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Site & Board Matters => About This Site => Topic started by: Ray on January 26, 2012, 04:13:35 am

Title: Perspective Revisited
Post by: Ray on January 26, 2012, 04:13:35 am
There has recently been a bit of controversy on 'perspective' in another couple of threads, one entitled, 'This puzzling business of 35mm lens equivalent", and the other 'Does a photo give spatial information?'

There seems to be a mantra that perspective is related only to distance beteen the observer (photographer) and subject, which is difficult to break through. It's almnost like a religion.

I really think the matter should be sorted out. There seems to be so much confusion on the issue. Below I'm attempting to enumerate all the relevant points that have occurred to me during the recent discussion in those threads I mentioned above..

(1) There's a mathematical and geometric definition of perspective which is useful and necessary for all sorts of drafting and sketching and the creation of computerised programs to convert 3-D into 2D, or to stitch images together etc etc.

(2) There's a human experience of perspective which may be at odds with that scientific definition of perspective.

(3) There's a reason for the discrepancy. Human vision is enormously complicated. Simple geometric rules cannot encompass it.

(4) I'm reminded of the current controversy over Mark Dubovoy's article in which he claims that 'Everything Matters'. This is relevant to the discussion on perspective, as experienced in the human mind.

Mark's point, as I understood it, was that seemingly trivial details can have a surprisingly significant effect. We shouldn't ignore them.

(5) Those who claim adamantly that perspective can only be changed by a change in distance to subject should make clear that they are referring to a mathematically and geometrically abstract defintion which does not necessarily encompass the human experience which is plain and simply, and unavoidably, biased in accordance with its own sense of brain-wired perspective.

(6)The classic example of proof for the statement that perspective cannot be altered without a change in position, is the cropping of a wide-angle shot to the same angle of view as a telephoto shot.

The adherents of this simplistic approach to perspective will point out that the cropped wide-angle shot will have the same perspective as the telephoto shot, as evidenced by the same broad size and shape of objects and angles apparent in both shots.

However, those of us who don't lack a subtle apprehension of detail, will notice that the cropped wide-angle shot is a bit fuzzy and lacking in detail, compared with the telephoto shot, which is sharp and clear.

Such indistinctness in the cropped wide-angle shot is indicative of greater distance. Clarity and sharpness, or to quote Mark Dubovoy, hyper-realism, is indicative of closeness.

Great painters realise this fact. If you want to depict something as being rather distant, the last thing you do is make it sharp and detailed, which a telephoto lens does.

I could go on, but I'll leave it at that for the sake of brevity.
Title: Re: Perspective Revisited
Post by: 32BT on January 26, 2012, 04:28:00 am
Any links to the original threads? This seems to make little sense by itself. Lenses usually are sharper when cropped, but you lose some resolution. Whether one compensates the other?

First thing that comes to mind:

A lens has a curved surface. If that curvature is exactly spherical and conforms to its field-of-view then (and only then) is there a direct relation between perspective and cropping, and will it be relatively simple to mathematically correct and stitch. But most modern lenses aren't spherical and cheap lenses may even have distortions that could be visible to the casual observer. In a fish-eye lens there is usually less need to compensate distortions, so it is better suited to mathematical corrections where field-of-view remains in a known state.

Is that the type of issues that the threads where referring to?
Title: Re: Perspective Revisited
Post by: Jim Pascoe on January 26, 2012, 05:21:56 am
Hi Mark

I was involved in a very small way in the thread you mention, and as I remember it the original point was about using lenses on different format sensors and how that related to the apparent perspective.  I think one of the points was for example if you used a 25mm lens on a MFT camera to photograph a face so that the face fairly well filled the frame, then the nose would be a bit too big looking in the picture, caused by perspective and the fact that we are too close to the subject.  If you then put the 25mm lens on a FF camera and took the picture from the same position (of course you would have a wider field of view), and then cropped the resulting picture to show the same size face, then the pictures would effectively look identical.  This was essentially what was being said.  And I think that is right.  This simple test basically proves that perspective is affected by distance to the subject and not the focal length of the lens.  Of course a wider lens will show much more of a scene and may artificially give a much greater sense of 'depth', but for a subject that is central in the image and could therefore be cropped down, the perspective is unchanged.

The previous thread seemed to degenerate into talking about lots of different things which might loosely be described as being about perspective, but if terms are used loosely enough they can be used to mean almost anything to different people.  One example might be the term 'aerial perspective', which I think might be used to describe the gradual reduction in clarity of distant objects in a landscape photograph caused by atmospheric haze. In a wide angle shot this will give a further feel about how far away distant objects are and can be quite effective.  But even this will be unaffected if you crop down a wide shot to the equivalent of a tele shot.  As long as both lenses are focussed on the same distant point the 'fuzziness' will be the same.  Of course this is an extreme example and obviously resolution and focussing errors would be at play here too.

But I think the sort of perspective originally being discussed in the earlier thread was concerned with geometric perspective, and that is only affected by distance from lens to subject.

In my opinion.

Jim

PS Having just re-read another part of one of the mentioned threads, I think you are also stating something that is very obvious about 'apparent' perspective.  So let us say that I shoot a group of distant building with a 200mm lens so that the buildings pretty much fill the image.  If I show you that picture you have no real way of knowing how far away they are unless I tell you the focal length of the lens and you could then make an educated guess.  If I also take a picture with a 21mm lens from the same spot and show that to you, it will become immediately obvious that the buildings are quite a long way off because you can see everything that is between the lens and the buildings.  I think that is what you mean by perspective - am I right?   But that is a different scene with a different lens, and if you crop down they will look the same, and if you could project outwards the lines from the 200mm shot you would get the same perspective.
Title: Re: Perspective Revisited
Post by: Bart_van_der_Wolf on January 26, 2012, 05:29:59 am
Any links to the original threads? This seems to make little sense by itself.

I agree, people should be clear about what they are referring to, unless their intention is to start all over.

This is what sparked Ray's urge to start this new thread:
Does a photo give spatial information (the nose job)? (http://www.luminous-landscape.com/forum/index.php?topic=61388.0)
and presumably
This puzzling business of "35mm lens equivalent" (http://www.luminous-landscape.com/forum/index.php?topic=61222.0)

Cheers,
Bart
Title: Re: Perspective Revisited
Post by: Ray on January 26, 2012, 06:42:33 am
Fred,
That's an excellent reiteration of the mantra that only position affects perspective. However, I'm speaking from the human perspective.

There are lots of issues other than 'real'  distance that influence the sense of perspective in the human brain. I mentioned one of them, specifically resolution, which you completely ignored.

Do I have to repeat that sharp objects appear closer, to the human brain, than fuzzy and indistinct objects, despite the geometry.

Do I need to mention that large objects visible in the foreground tend to make other objects that are further away, seem even further away than they actually are?

Do I also need to mention if those larger objects in the foregrounds are cropped out of the image, the perspective of the remaininmg image has changed?

Fred, you are just repeating a false notion which you have been taught but which you have never questioned, it seems to me.
Title: Re: Perspective Revisited
Post by: 32BT on January 26, 2012, 06:54:29 am
I agree, people should be clear about what they are referring to, unless their intention is to start all over.

This is what sparked Ray's urge to start this new thread:
Does a photo give spatial information (the nose job)? (http://www.luminous-landscape.com/forum/index.php?topic=61388.0)
and presumably
This puzzling business of "35mm lens equivalent" (http://www.luminous-landscape.com/forum/index.php?topic=61222.0)

Cheers,
Bart

Okay, thanks.

Perhaps the following thought may be of interest:

A 180degr fisheye is usually indicated as 8mm, but 180degr field of view has a 0mm focal point. So, I generally read the 8mm as an indication of the distance to the plane of focus. If you attach the lens to a cropped sensor, at the correct distance, it will capture a sharp image of a smaller field of view, but if you attach the lens closer to the cropped sensor, it may still capture 180degr FOV, but the image may well be fuzzy.

I believe issues like these are far more relevant than any psycho-accoustics that may be at play.
Title: Re: Perspective Revisited
Post by: j-land on January 26, 2012, 07:01:37 am
There has recently been a bit of controversy on 'perspective' in another couple of threads, one entitled, 'This puzzling business of 35mm lens equivalent", and the other 'Does a photo give spatial information?'

There seems to be a mantra that perspective is related only to distance beteen the observer (photographer) and subject, which is difficult to break through. It's almnost like a religion.

I really think the matter should be sorted out. There seems to be so much confusion on the issue.

Agreed, many are confused, but often due to technical misconceptions or semantic differences.

Quote
Below I'm attempting to enumerate all the relevant points that have occurred to me during the recent discussion in those threads I mentioned above..

(1) There's a mathematical and geometric definition of perspective which is useful and necessary for all sorts of drafting and sketching and the creation of computerised programs to convert 3-D into 2D, or to stitch images together etc etc.

It's also fundamental to the creation of images by thing we call "photography".

Quote
(2) There's a human experience of perspective which may be at odds with that scientific definition of perspective.

Human visual perception is in many ways very complex and not as "objective" as imaging by a machine. Witness the many optical illusions that play on the brain's processing of visual information and the fact that we exist and process things in a temporal environment (mentioned, I think, in another thread). Of course one's psychological state at any given moment will affect perception, i.e. when under stress or in a "fight or flight" situation, your brain's visual filtering will be quite different than when you're relaxed and taking in the scenery on a sunny day. Are you using the term "perspective" to mean "perception"?

Quote
(3) There's a reason for the discrepancy. Human vision is enormously complicated. Simple geometric rules cannot encompass it.

Sure...

Quote
(4) I'm reminded of the current controversy over Mark Dubovoy's article in which he claims that 'Everything Matters'. This is relevant to the discussion on perspective, as experienced in the human mind.

Please elaborate...

Quote
Mark's point, as I understood it, was that seemingly trivial details can have a surprisingly significant effect. We shouldn't ignore them.


(5) Those who claim adamantly that perspective can only be changed by a change in distance to subject should make clear that they are referring to a mathematically and geometrically abstract defintion which does not necessarily encompass the human experience which is plain and simply, and unavoidably, biased in accordance with its own sense of brain-wired perspective.

Those claims about perspective are (largely, in the thread you mentioned) addressing the questions about equivalent focal length which is fundamentally a geometric phenomenon. Besides, the geometric definition is not so abstract - when you stand looking down some railway tracks or at the bottom of a tall building, it's quite apparent that the parallel lines converge (in my experience, anyway). I would propose that it was observation and curiosity about human vision that led to the development those definitions of perspective in the first place. I don't quite know what you have in mind and are vaguely alluding to, or is it that you haven't clearly formed your ideas yet?

Quote
(6)The classic example of proof for the statement that perspective cannot be altered without a change in position, is the cropping of a wide-angle shot to the same angle of view as a telephoto shot.

The adherents of this simplistic approach to perspective will point out that the cropped wide-angle shot will have the same perspective as the telephoto shot, as evidenced by the same broad size and shape of objects and angles apparent in both shots.

However, those of us who don't lack a subtle apprehension of detail, will notice that the cropped wide-angle shot is a bit fuzzy and lacking in detail, compared with the telephoto shot, which is sharp and clear.

Such indistinctness in the cropped wide-angle shot is indicative of greater distance. Clarity and sharpness, or to quote Mark Dubovoy, hyper-realism, is indicative of closeness.

It seems like you're observing differences in resolution where, assuming the wide angle and telephoto shots are made using the same sensor or film emulsion, the cropped part of the wide angle image needs to be enlarged to match it to the telephoto image, resulting in loss of detail. If I change my telephoto lens to another lens of the same focal length that isn't as sharp and has more flare, is the fuzzy and less distinct photograph from the second lens more "distant" than that from the first, even though they encompass exactly the same field of view? What are you getting at with your use of the word "distance"?

Quote
Great painters realise this fact. If you want to depict something as being rather distant, the last thing you do is make it sharp and detailed, which a telephoto lens does.

Sounds like you're talking about aerial perspective, where distant things lose contrast (and possibly resolution) due to scattering of light in the atmosphere. It is indeed a device used by artists to give the perception of depth in an image. Exactly the same thing happens in photographs. If you're an a hill overlooking a smoggy city the bushes in the foreground will be contrastier than the distant building. When you switch from wide angle to telephoto, the telephoto has to look through exactly the same depth of smoggy air as the wide angle and tje distant building will show the same loss of contrast. Any differences will be due to characteristics of the different lenses or the aforementioned effects of enlargement. It's possible to make a photograph with just about any lens where the distant things are sharp and the foreground is out of focus, so detail or lack thereof is not necessarily indicative of distance. In art and 2-dimensional imaging there are many devices used to give cues for distance, many of which have been touched upon in the threads you mention. It's the ability to play around with these devices in sometimes contradictory ways that has resulted in interesting works of art. There are many books about the subject. If you want to call this "perspective", fine, but don't expect any consensus on that definition.

Quote
I could go on, but I'll leave it at that for the sake of brevity.

Good idea.
Title: Re: Perspective Revisited
Post by: Ray on January 26, 2012, 07:16:46 am

PS Having just re-read another part of one of the mentioned threads, I think you are also stating something that is very obvious about 'apparent' perspective.  So let us say that I shoot a group of distant building with a 200mm lens so that the buildings pretty much fill the image.  If I show you that picture you have no real way of knowing how far away they are unless I tell you the focal length of the lens and you could then make an educated guess.  If I also take a picture with a 21mm lens from the same spot and show that to you, it will become immediately obvious that the buildings are quite a long way off because you can see everything that is between the lens and the buildings.  I think that is what you mean by perspective - am I right?   But that is a different scene with a different lens, and if you crop down they will look the same, and if you could project outwards the lines from the 200mm shot you would get the same perspective.

Perhaps the confusion results from a lack of a clear definition of perspective. The general definition of perspective is perhaps too narrow in the sense that it only addresses distances and angles.

It seems that resolution, texture, Dof, FoV have no bearing on this geometric definition of perspective that most people on this forum seen to slavishly accept, but I suspect their eyes don't.
Title: Re: Perspective Revisited
Post by: Walter_temp on January 26, 2012, 07:40:48 am
If you try to cover all traps and pitfalls of human vision: Good luck!
The original thread started with the misconception of small sensor sizes and the big nose effect, which was settled in the discussion (I thought). This part was "correlation between sensor size, focal length and distance" for the picture taken by the camera.

Now you're trying to turn the whole thing upside down and - sorry to say - there are some open questions that will not be solved by you, neither any of us. The thing has been discussed a long time ago. Take a look into some books written by Andreas Feininger. He pointed out some issues where a picture and the human perception doesn't match. This one is easy to reproduce: Wide angle lens, person in front of you with arms aiming to camera. The camera will reproduce what the eye is seeing but our brain does have another picture in mind. If you remember such a scene you will not remember hand covering most of the "image" and head reduced to a pinhead. You will remember a person with arms and hands stretched to you and all proportions will be just fine ... in your imagination. But: If you are able to reproduce the scene and just look what your eyes are covering ... well ... this will resemble the picture taken by the camera (mostly).

This is not new knowledge at all. Sometimes I'm just baffled that there are photographers who aren't aware that there is a lot of difference between "eye sight" and "mind sight". There are misconceptions about what the eye sees (as in "lens and perspective") and the inner representation of the world we think we are seeing.

Another well known example is the "Moon illusion" and i think we've all been there at least once: Take a picture of the moon near horizon with a "normal" lens (which most people insist to be equivalent to human sight, another gross misconception IMO) and the moon in the picture will be look like riduced and not giving the perception you had at all.

So what? The eye is a liar and the ear, too! That's what Prof. Pauschinger (RIP) shouted in one of his remarkable entertaining lectures.

And this defines a part of the photographer's job: To know the difference between human perception in real live and the things a camera can do and take the picture that shows what *you* like to show. Beginners will fail here.
Well known example is beginner's question why a picture of a little bird sitting in the tree (bird just covering a little fraction of the picture) just looks awful and the bird is hardly recognizable at all and the picture is not showing what was seen "by the eye".


So: What's this all about? Do you want to lecture us about things as old as photography and partly older?

Ciao, Walter
Title: Re: Perspective Revisited
Post by: j-land on January 26, 2012, 07:46:34 am
Ok, you want to talk about distance in pictorial representation as a general definition of perspective. If there's a photo of a landscape with the distant things sharp and an out of focus tree in the foreground, the tree does not look distant despite being out of focus because of it's relative size and it overlaps the things in the background. If the tree is chopped down and another picture is taken, the spacial perception of those distant objects may be different without the tree as a relative gauge - fine. As I mentioned previously, there are many devices that can be used in an image to affect the viewer's perception of distance. This is all pretty standard stuff that has been known and used in art for centuries. There is really nothing fundamentally "false" about what Fred said, and much of what you are asserting isn't false either, but I think if you clarified your semantics a little, there would be a lot less confusion all around.

Fred,
That's an excellent reiteration of the mantra that only position affects perspective. However, I'm speaking from the human perspective.

There are lots of issues other than 'real'  distance that influence the sense of perspective in the human brain. I mentioned one of them, specifically resolution, which you completely ignored.

Do I have to repeat that sharp objects appear closer, to the human brain, than fuzzy and indistinct objects, despite the geometry.

Do I need to mention that large objects visible in the foreground tend to make other objects that are further away, seem even further away than they actually are?

Do I also need to mention if those larger objects in the foregrounds are cropped out of the image, the perspective of the remaininmg image has changed?

Fred, you are just repeating a false notion which you have been taught but which you have never questioned, it seems to me.
Title: Re: Perspective Revisited
Post by: Ray on January 26, 2012, 07:59:11 am
Human visual perception is in many ways very complex and not as "objective" as imaging by a machine. Witness the many optical illusions that play on the brain's processing of visual information and the fact that we exist and process things in a temporal environment (mentioned, I think, in another thread). Of course one's psychological state at any given moment will affect perception, i.e. when under stress or in a "fight or flight" situation, your brain's visual filtering will be quite different than when you're relaxed and taking in the scenery on a sunny day. Are you using the term "perspective" to mean "perception"?


Let's just address this issue above. Optical illusions certainly exist, and they are quite amazing. But one essential point about these illusions is that they seem to apply to everyone.

If you wish to make the point that various factors other than mere distance, that might affect the human brain's perception of perspective, are purely an illusion, then we are into a branch of philosophy rather than photography, which I'm quite willing to discuss.

Are you making the point that the extreme and contrived examples that trick the eye, such as people simultaneously walking up and down a staircase which is on the same level, are what we normally see in photographic images? In other woerds, everything is an illusion except the geometry of perspective?
Title: Re: Perspective Revisited
Post by: j-land on January 26, 2012, 08:39:32 am
My point, somewhat agreeing with you, is that tone and contrast (and not just pure geometry), among numerous other things, can affect pictorial perception of distance, and we often use these effects in images through manipulations as simple as burning and dodging. By optical illusions I don't mean just Escheresque creations, but examples like  simultaneous contrast (http://en.wikipedia.org/wiki/Contrast_effect) show how our brains have evolved to process and perceive visual information (at some point these things were probably useful to distinguish our potential prey and predators in the forest). These concepts can be used in creating images and "fool" the eye in order to exaggerate or force a certain illusion of spacial relations in a photograph. In some ways I think you're preaching to the converted, but just need to clarify the terms your using and what they mean to you. I would question the value of bringing this aspect of "perspective" into the topic of equivalent focal lengths... and would not be so derisive of geometric perspective in that context.

Let's just address this issue above. Optical illusions certainly exist, and they are quite amazing. But one essential point about these illusions is that they seem to apply to everyone.

If you wish to make the point that various factors other than mere distance, that might affect the human brain's perception of perspective, are purely an illusion, then we are into a branch of philosophy rather than photography, which I'm quite willing to discuss.

Are you making the point that the extreme and contrived examples that trick the eye, such as people simultaneously walking up and down a staircase which is on the same level, are what we normally see in photographic images? In other woerds, everything is an illusion except the geometry of perspective?
Title: Re: Perspective Revisited
Post by: Bart_van_der_Wolf on January 26, 2012, 09:09:44 am
Perhaps the confusion results from a lack of a clear definition of perspective. The general definition of perspective is perhaps too narrow in the sense that it only addresses distances and angles.

Dear Ray,

Perspective is only about distances and angles, as seen from a unique vantage point (the perspective point or entrance pupil of a lens in the case of photography). The perception of perspective can be confused for an observer by various added distortions.

Quote
It seems that resolution, texture, Dof, FoV have no bearing on this geometric definition of perspective

Correct.

Quote
... that most people on this forum seen to slavishly accept, but I suspect their eyes don't.

Optical illusions do not change geometry, only the perception of it (if at all). Just because you close your eyes, doesn't make the scene go dark for most of the other observers...

Cheers,
Bart
Title: Re: Perspective Revisited
Post by: Ray on January 26, 2012, 09:18:12 am
I would question the value of bringing this aspect of "perspective" into the topic of equivalent focal lengths... and would not be so derisive of geometric perspective in that context.


I'm not trying to be derisive. I'm simply searching for clarity on the issue. I could probably accept a definition of perspective along the lines, "Perspective is affected only by distance to subject. Any other factors which might suggest otherwise, exist only in the mind of the viewer and are pure illusion."

The second part needs to be stressed. The implication is, the extreme examples of perspective distortion that most of us have seen, such as two normal people in a room who look hugely different in size, are just that; extreme examples of a distortion that is normal in everyday vision. In other words, everything we see is a distortion to some degree, according to the geometric rules of perspective.
Title: Re: Perspective Revisited
Post by: Ray on January 26, 2012, 09:56:15 am
"Perspective is affected only by distance to subject"

Noooo..... Arrghh.

On the other hand, I might be a little worried about accepting a concept that everything I see is a distortion of reality in relation to the purity of geometry, when the science of geometry is itself a product of the human mind, and therefore subject to distortion.
Title: Re: Perspective Revisited
Post by: Ray on January 26, 2012, 10:17:48 am
;D Then you have no other option than to ask the gods themselves. (the responsible "thing" whatever it is of this universe and its laws).

Well, I do have the option of just ignoring the theoretical rules of geometry and doing what I damned well please.  ;D
Title: Re: Perspective Revisited
Post by: Bart_van_der_Wolf on January 26, 2012, 10:45:00 am
Well, I do have the option of just ignoring the theoretical rules of geometry and doing what I damned well please.  ;D

The rules of projective geometry are not a theory, they are based on axioms (http://en.wikipedia.org/wiki/Projective_geometry#Axioms_of_projective_geometry).
If you don't agree, well, how about ignoring the 'theory of gravity'. I wish you a speedy recovery ;)

Cheers,
Bart
Title: Re: Perspective Revisited
Post by: C Debelmas on January 26, 2012, 01:47:44 pm
I'm afraid to say that you are comparing pears and apples.
Forget the word perspective which is confusing because it has many meanings.
Just consider the way a 3D object is projected on 2D surface (the film or the sensor). The shapes that you obtain on the 2D surface is only affected by the "point of view", i.e. the position of the 2D surface in respect to the 3D object. As you have to consider lenses in photography (you need lense to create a projection on the film or the sensor), you may use wide angle lenses or telephoto lenses. Then what is at sake is that when you change the focal length you don't change the shape on the 2D surface (expect if you consider the possible croping and the possible distorsion of the lenses).
Then consider the "feeling of distance" that you may have from your position with respect to the 3D object : if you rely on the shape of the 3D object projected on the 2D surface, you will have no clue about the distance. If you have other informations, like the color of the object or the blur then you may have a clue on the actual distance.
One thing is the geometry of the 2D shape, another thing is the aspect (color, blur) of the 2D shape.
Title: Re: Perspective Revisited
Post by: 32BT on January 26, 2012, 02:33:17 pm
Would be funny to sent a Lytro picture of a 2D image to Lytro and ask them why the refocusing doesn't work…

Title: Re: Perspective Revisited
Post by: Ray on January 27, 2012, 11:52:19 am
The rules of projective geometry are not a theory, they are based on axioms (http://en.wikipedia.org/wiki/Projective_geometry#Axioms_of_projective_geometry).
If you don't agree, well, how about ignoring the 'theory of gravity'. I wish you a speedy recovery ;)

Cheers,
Bart

C'mon Bart,
You haven't thought that through, have you!  ;D One doesn't need to know anything about the theory of gravity in order to negotiate the problems of gravity on a personal level. There were circus performers long before Isaac Newton came on the scene with his theory of gravity, who were able to perform amazing balancing acts without any theoretical knowledge of gravity whatsoever.

More likely, I would probably need a speedy recovery if I were to spend too much time contemplating the mystery of the gravitational force as I walked down the street. I might fall into a pot-hole through not paying attention to what I was doing.  ;D

As regards axioms, it's axiomatic to me that all theories are a construct of the human brain and imagination.They represent models of reality which are usually found to be, eventually, either a bit wrong or completely wrong.

Astrophysicists are currently having a hard time explaining why 95% of the matter and energy in the universe is invisible and undetectable. The euphemistic term is Dark Matter.

There's a consensus of opinion amongst Astronomers and Astrophysicists that Dark Matter and Dark Energy really does exist, despite the fact that not one particle has been discovered so far. On the other hand, there's a growing number of impatient scientists who think our theories of gravity may be wrong, and who have attempted to modify Newton's theories. The acronym for the alternative theory to Dark Matter is MOND (Modification Of Newtonian Dynamics).

Cheers!

Title: Re: Perspective Revisited
Post by: John Camp on January 27, 2012, 01:50:58 pm
Ray, you're essentially arguing nonsense, and I think you know it.

Of course the perception of perspective may differ from person to person, but the fact that my vision is 20-40 and you wear glasses to correct an astigmatism and the third guy is blind has nothing to do with the way a camera operates. All three of us could push the button on a camera, one after the the other, and we'd get the same photo. I doesn't depend on how we feel or what we think. The fact that there is fog in the valley doesn't change the lens. Perspective is determined by position, period. Perspective is not a philosophical concept, or an artificial construct, or negotiated in any way, it just is. (Unless, of course, you're a Republican, in which case we may need a new thread.)

What you're arguing is basically semantics. You're saying that because somebody somewhere may have misused the word "perspective," and their culture is as good as ours, then it's okay to say that perspective changes. No.
Title: Re: Perspective Revisited
Post by: Eric Myrvaagnes on January 27, 2012, 04:22:50 pm
Why should they? Why isn't the burden on you to make clear what you are referring to when you talk of perspective? Why is it that you own the definition of that word?


Because Ray is an ardent follower of that eminent philosopher Humpty Dumpty: “When I use a word,’ Humpty Dumpty said in rather a scornful tone, ‘it means just what I choose it to mean — neither more nor less.’" (Lewis Carroll)

Eric  ;D
Title: Re: Perspective Revisited
Post by: mouse on January 27, 2012, 05:42:53 pm
Because Ray is an ardent follower of that eminent philosopher Humpty Dumpty: “When I use a word,’ Humpty Dumpty said in rather a scornful tone, ‘it means just what I choose it to mean — neither more nor less.’" (Lewis Carroll)

Eric  ;D

In response to Ray, in another thread, I wrote:

I think I finally understand Ray's problem.

While the majority of photographers, painters, and others involved in creating or interpreting 2D images have adopted the standard, mathematically defined concept of (linear) perspective, Ray has created his own personal definition of perspective..  He has every right to do so, but it is neither legitimate nor rational for him to argue that the consensus definition is wrong while his is correct.  While Ray may continue to submit any amount of evidence in support of his personal definition, nothing he has written calls into question the legitimacy of standard definition nor suggests that his definition is more useful or appropriate.

But Humpty Dumpty said it better. ;)
Title: Re: Perspective Revisited
Post by: Ray on January 28, 2012, 12:48:26 am
Ray, you're essentially arguing nonsense, and I think you know it.

Of course the perception of perspective may differ from person to person, but the fact that my vision is 20-40 and you wear glasses to correct an astigmatism and the third guy is blind has nothing to do with the way a camera operates. All three of us could push the button on a camera, one after the the other, and we'd get the same photo. I doesn't depend on how we feel or what we think. The fact that there is fog in the valley doesn't change the lens. Perspective is determined by position, period. Perspective is not a philosophical concept, or an artificial construct, or negotiated in any way, it just is. (Unless, of course, you're a Republican, in which case we may need a new thread.)

What you're arguing is basically semantics. You're saying that because somebody somewhere may have misused the word "perspective," and their culture is as good as ours, then it's okay to say that perspective changes. No.

Not at all! You seem to have missed my point entirely, John. What I'm arguing makes total sense. I'm simply stating that using a different focal length of lens from the same position changes what's included or excluded in the scene and that exclusion or inclusion changes the sense of distance from the viewer to the various objects in the scene.

How anyone  could deny that fact beats me. Of course it's also true that, irrespective of lens used, if one crops or successfully stitches images so that the scenes do not differ with respect to what's included or excluded, in other words one arranges for the images to have the same angle of view, then perspective will broadly be the same, apart from the more subtle influences of greater resolution and clarity, or differences in Dof that one of the images may have.

Now the usual proof offered to support the theory that lens focal length has no bearing on perspective, is the above example of cropping a wide angle shot so that it has the same FoV as a longer focal length. This where the nonsense lies.

The fact that perspective is the same when one makes the angle of view the same, demostrates the principle of equivalent or effective focal length, which is why one of those threads that Bart linked to began with a confusion about lens FL equivalents on different camera formats and subsequently drifted into the issue of perspective because the equality of perspective evident at equal FoV and distance to subject, is the proof of the concept of equivalent or effective focal length, not proof that any focal length used, which is not effectively the same, provides equal perspective.

In other  words, a 30mm lens on an APS-C format really is equivalent to a 50mm lens on full-frame 35mm format, which really is equivalent to an 80mm lens on MF format, which really is equivalent to a 360mm lens on 8'x10' format (approximately, taking into account differences in resolution and DoF)).

How do we know that these different focal lengths of lens are all effectively the same, when used from the same position and direction with the appropriate format of camera? Because the perspective is the same.

How do we know perspective is the same using the same 'effective' focal length of lens?

Because our eyes tell us it's the same, when we compare the photos.

How do we know that changing the 'effective' FL of lens, at the same position, changes our sense of perspective?  Because our eyes tell us it has changed. Wide-angle shots make close objects appear closer, and distant object more distant. Didn't you know that?

Goodness gracious me!  ;D
Title: Re: Perspective Revisited
Post by: mouse on January 28, 2012, 01:33:49 am

Wide-angle shots make close objects appear closer, and distant object more distant. Didn't you know that?

Goodness gracious me!  ;D


Ray, Since you are so interested in what our eyes have to tell us,  I have an experiment for you to perform.  Secure the cardboard tube from the center of a roll of paper towels.  Go outside and find a distant object and a close object.  Hold the cardboard tube up to your eye and look at those two objects.  Now remove the tube (thus converting your narrow angle vision thru the tube to a wide angle vision).  Did the close object suddenly appear closer; or the distant object more distant?  If they did, you need to visit your ophthalmologist.
Title: Re: Perspective Revisited
Post by: Ray on January 28, 2012, 02:49:31 am
Ray, Since you are so interested in what our eyes have to tell us,  I have an experiment for you to perform.  Secure the cardboard tube from the center of a roll of paper towels.  Go outside and find a distant object and a close object.  Hold the cardboard tube up to your eye and look at those two objects.  Now remove the tube (thus converting your narrow angle vision thru the tube to a wide angle vision).  Did the close object suddenly appear closer; or the distant object more distant?  If they did, you need to visit your ophthalmologist.

Exactly true! Good analogy! The perspective was the same because the focal length was the same, that is, the focal length of my eyes did not change simply because I was peering through a cardboard tube.

Camera lenses are not cardboard tubes. They are lenses, as our eyes are lenses, but different in some ways of course. If instead of using a cardboard tube, I'd used my D700 with a standard 50mm lens, I'd have got a similar effect, but not with a different focal length on my D700.

Let me try to explain what I think may be happening in this issue by providing additional clarification on this aspect of inclusion and exclusion.

One of the problems that scientific enquiry faces, which presents a problem in the formulation of theories and the confirmation or falsification of theories, is selection bias. One can collect a huge quantity of data on a particular subject, but the data one chooses to include or exclude will either confirm or refute the results one may hope to achieve, or the theory one is trying to either refute or substantiate

However, there's an additional problem because bias by its very nature is something we are not fully aware of. To be aware of one's biases is to be unbiased.
To  behave in a biased manner in science, despite being aware one is biased on a specific issue, is tantamount to scientific fraud. Through a process of careful exclusion of specific data which doesn't support one's hypothesis, and the inclusion only of the data which does support one's hypothesis, one can prove or disprove almost anything.

So let's apply the above principle to this issue of the sense of perspective that a viewer experiences, when viewing an image of a scene through different focal lengths of lenses, from the same position.

If one wishes to test this in a scientific manner, one should take a number of shots from the same position actually using different focal lengths of lenses, then compare the images.

When I do this it is clear to me that a wide angle shot produces a different sense of perspective to a telephoto shot. However, since I have a fair understanding of this principle of 'selection bias', I know that I can turn these results on their head by excluding data from the wide-angle shot that gives the impression that the perspective is different. I do this by cropping out the offending data.

If I have two sets of data which are different, one set being larger than the other, and I exclude from the larger set all the data that is different to the smaller set, then I'm obviously left with two identical sets of data.

Such is the proof that focal length has no bearing on perspective.

Really! Pull the other leg.  ;D


Title: Re: Perspective Revisited
Post by: 32BT on January 28, 2012, 03:00:37 am
It once occurred to me that the human eye is more akin to a fish-eye perspective than it is to a rectilinear perspective, even though we very much need the latter to interpret 2D images...
Title: Re: Perspective Revisited
Post by: Ray on January 28, 2012, 04:05:34 am
It once occurred to me that the human eye is more akin to a fish-eye perspective than it is to a rectilinear perspective, even though we very much need the latter to interpret 2D images...

My widest-angle lens is the rectilinear Nikkor 14mm (as in 14-24 zoom). When I raise that to my eyes, then compare the scene without camera, doing my best to stare at the same object in the centre of the scene, I find that there are peripheral, but broadly recognisable, objects in my vision which are outside the FoV shown in the 14mm lens, implying that my angle of vision is indeed wider than 14mm (full-frame 35mm equivalent).

The problem is in the definition of peripheral vision, which I suspect is the reason why there are so many variants on the true angle-of-view of human vision.

It seems very clear that the angle of focussed view in human vision is very narrow indeed. As the angle away from that very narrow focussed view increases, the objects lose more and more detail to the point where only sudden movement in the scene is detectable.

One of the problems in lens design of the wide angle, appears to be correcting for 'volume anamorphosis', as mentioned by Bart. This can make objects near the edges of the frame appear odd, distorted and larger in some way than the eye/brain perceives they should be.

However this is a separate issue from the general effect that wide-angle lenses make objects more distant, and telephoto lenses make objects appear closer.

Title: Re: Perspective Revisited
Post by: Guillermo Luijk on January 28, 2012, 04:34:53 am
How do we know that changing the 'effective' FL of lens, at the same position, changes our sense of perspective?  Because our eyes tell us it has changed. Wide-angle shots make close objects appear closer, and distant object more distant. Didn't you know that?

Sorry Ray, but I am not able to understand the point of this entire thread. There are some facts that we'll all agree:


So, where is the discussion?  ??? all this just seems a semantic discussion to me.

(http://www.guillermoluijk.com/article/perspective/cocina.jpg)

This is my kitchen shot from a tripod without changing the direction of observation. Of course the visual perception of the 10mm frame is different to that of the 17mm and 20mm shots, but nobody denies this, one picture shows more things than the other!. And of course the cropped content (geometrical perspective) is the same in all three shots. So?

Regards
Title: Re: Perspective Revisited
Post by: 32BT on January 28, 2012, 04:50:18 am

However this is a separate issue from the general effect that wide-angle lenses make objects more distant, and telephoto lenses make objects appear closer.


Okay, got that. Sorry for reiterating.

Title: Re: Perspective Revisited
Post by: Ray on January 28, 2012, 05:34:05 am
.... we can only change the orientation (direction of observation) of a given shot, never its perspective in terms of what is seen and what is hidden.[/li]
[/list]

So, where is the discussion?  ??? all this just seems a semantic discussion to me.


The perspective of what precisely is hidden?? This is the core of the nonsense. You are stating that the perspective of that which isn't there, which doesn't exist, which isn't in the photo, has not changed.

Quote
This is my kitchen shot from a tripod without changing the direction of observation. Of course the visual perception of the 10mm frame is different to that of the 17mm and 20mm shots, but nobody denies this, one picture shows more things than the other!. And of course the cropped content (geometrical perspective) is the same in all three shots. So?

So, if you make the stuff in your 10mm shot hidden (by cropping), your microwave looks closer to the viewer, whoever that viewer may be.

This is not semantics. It's an actual and real effect of human vision.

Now, if you wish to make the point that perspective has nothing to do with the observer and his sense of distance to objects, then we are getting into even greater nonsense.  ;D
Title: Re: Perspective Revisited
Post by: Guillermo Luijk on January 28, 2012, 05:50:59 am
The perspective of what precisely is hidden?? This is the core of the nonsense. You are stating that the perspective of that which isn't there, which doesn't exist, which isn't in the photo, has not changed.

No Ray, not the perspective of what is hidden, the perspective of what is seen, which is the only perspective that can be judged on both images. All this seems so obvious to me that I wonder if you are just making a joke.


So, if you make the stuff in your 10mm shot hidden (by cropping), your microwave looks closer to the viewer, whoever that viewer may be.

Yes, you get an image where the microwave looks closer; in fact you get the same image as you got with the 22mm. Again: are you asking all these things seriously?  ???

I think you are not differentiating geometrical perspective (the result of a linear projection onto a plane, no matter how much is cropped) from the observer's image perception. Both concepts run in parallel, and are 100% compatible and explainable.
Title: Re: Perspective Revisited
Post by: Ray on January 28, 2012, 06:13:13 am
No Ray, not the perspective of what is hidden, the perspective of what is seen, which is the only perspective that can be judged on both images. All this seems so obvious to me that I wonder if you are just making a joke.


Yes, you get an image where the microwave looks closer; in fact you get the same image as you got with the 22mm. Again: are you asking all these things seriously?  ???


If what is seen in both images is different, how can the perspective be the same. It is you who is making a joke, right?  ;D

Guillermo, you only get part of the same image, if you crop. Also, if you crop a 10mm shot to the FoV of a 22mm shot, you've effectively changed focal length of lens. That change in focal length has made your microwave look closer. I'm glad you agree.  ;D
Title: Re: Perspective Revisited
Post by: Guillermo Luijk on January 28, 2012, 06:29:40 am
If what is seen in both images is different, how can the perspective be the same. It is you who is making a joke, right?  ;D

As I said and you said no, this is all about semantics. The geometrical perspective is the same, because the linear projection is the same and takes place under the same conditions (distance, direction of observation...). The visual perception is not the same since both images are different.

The longer you insist in mixing both conceps (the geometrical perspective which solely depends on subject distance, vs the visual perception which depends both on subject distance and FOV), the longer it will take you to realize we are talking about the same thing and we all agree.


I'm glad you agree.  ;D

That's why I don't understand why you opened this thread, if we all agree, you must be kidding or something.
Title: Re: Perspective Revisited
Post by: Eric Myrvaagnes on January 28, 2012, 09:39:42 am
In Ray's earlier posts on this general topic he kept referring to "perspective" in a way that got many of us riled up. At some point he started using (occasionally, but I don't think consistently) the phrase "perception of perspective." It seems to me that he has been arguing for his own idea of "perception of perspective" all along and not talking about the generally understood mathematical notion of "perspective."

I will readily admit that certain very wide angle images can confuse the brain's interpretation of distances and sizes, but so can all the other optical illusions that Bernard posted a link to.

But none of that changes the generally understood meaning of "perspective," unless you take Humpty Dumpty's view of words.

Eric
Title: Re: Perspective Revisited
Post by: Isaac on January 28, 2012, 12:45:02 pm
The longer you insist in mixing both conceps (the geometrical perspective which solely depends on subject distance, vs the visual perception which depends both on subject distance and FOV), the longer it will take you to realize we are talking about the same thing and we all agree.

Ray has been refusing to acknowledge that distinction and that agreement for days -

Now, what you pointed, the subjective factors that actually fool the eyes (atmosphere is one) and alter our perception of the perspective is correct. So both, what you are pointing, and the geometry  are correct and not in contradiction at all.

Ray is a great quarreller - he wishes to quarrel, he does not wish to understand - I dare say he would quarrel with Brunelleschi.
Title: Re: Perspective Revisited
Post by: Wayne Fox on January 28, 2012, 04:43:39 pm
Exactly true! Good analogy! The perspective was the same because the focal length was the same, that is, the focal length of my eyes did not change simply because I was peering through a cardboard tube.

Camera lenses are not cardboard tubes. They are lenses, as our eyes are lenses, but different in some ways of course. If instead of using a cardboard tube, I'd used my D700 with a standard 50mm lens, I'd have got a similar effect, but not with a different focal length on my D700.

Let me try to explain what I think may be happening in this issue by providing additional clarification on this aspect of inclusion and exclusion.

One of the problems that scientific enquiry faces, which presents a problem in the formulation of theories and the confirmation or falsification of theories, is selection bias. One can collect a huge quantity of data on a particular subject, but the data one chooses to include or exclude will either confirm or refute the results one may hope to achieve, or the theory one is trying to either refute or substantiate

However, there's an additional problem because bias by its very nature is something we are not fully aware of. To be aware of one's biases is to be unbiased.
To  behave in a biased manner in science, despite being aware one is biased on a specific issue, is tantamount to scientific fraud. Through a process of careful exclusion of specific data which doesn't support one's hypothesis, and the inclusion only of the data which does support one's hypothesis, one can prove or disprove almost anything.

So let's apply the above principle to this issue of the sense of perspective that a viewer experiences, when viewing an image of a scene through different focal lengths of lenses, from the same position.

If one wishes to test this in a scientific manner, one should take a number of shots from the same position actually using different focal lengths of lenses, then compare the images.

When I do this it is clear to me that a wide angle shot produces a different sense of perspective to a telephoto shot. However, since I have a fair understanding of this principle of 'selection bias', I know that I can turn these results on their head by excluding data from the wide-angle shot that gives the impression that the perspective is different. I do this by cropping out the offending data.

If I have two sets of data which are different, one set being larger than the other, and I exclude from the larger set all the data that is different to the smaller set, then I'm obviously left with two identical sets of data.

Such is the proof that focal length has no bearing on perspective.

Really! Pull the other leg.  ;D



Ray,

with apologies and please don't take any offense, but on this subject you really should leave it alone.  You just can't get past the idea that distortion is not perspective.  The point is when you remove the tube, you see more, but the perspective of the objects you saw with only the tube doesn't change.  The only way to change that is move your position in relation to those objects.  Make a framing guide from a piece of matt board and find a set of telephone poles receding in the distance.  Frame an image with the rectangle and note the perspective of the poles to each other.  When you remove the guide, that doesn't change.  You see more, and that's all focal length on a lens does.  Any other effect(distortion) is unintentional ( and now almost all of us correct it with software).

This insistence on this concept (you do realize that may be the ONLY one that believes this) reflects poorly on your credibility.

Title: Re: Perspective Revisited
Post by: Ray on January 29, 2012, 02:01:19 am
The longer you insist in mixing both conceps (the geometrical perspective which solely depends on subject distance, vs the visual perception which depends both on subject distance and FOV), the longer it will take you to realize we are talking about the same thing and we all agree.



I wasn't aware that I was mixing up these two concepts. I've never claimed that perspective distortion due to changes in distance are identical and interchangeable with perspective distortion due to the use of a wide angle or telephoto lens. I recognise they are different effects, but effects nevertheless which relate to the relative distances of objects to viewer. However, when those two effects are combined, one gets the greatest amount of perspective distortion, as in the use of a wide-angle lens from close up.

Now I understand the argument if one takes a portrait from a certain close distance using a wide angle lens which captures the whole face and some surrounding background detail, then compares that shot with another shot of the same face taken from the same distance using a telephoto lens which can capture only the nose, then the shape and perspective distortion of the nose only, will be the same in both shots, but only if one crops the nose in the wide-angle shot. That's the essential point I'm trying to get across.

If one doesn't crop the nose in the wide-angle shot to the same FoV as the telephoto shot, the perspective distortion from the wide-angle lens has an additive effect, making the nose appear huge in relation to the rest of the face.

So, my question to you, Guillermo, is do you agree with the following statements, and if not why not?

(1) Without the rest of the face visible, the size of the nose in relation to the eyes and ears, cannot be gauged in the telephoto shot. We cannot determine whether or not the nose is unusually large or not.

(2) If we crop the wide-angle shot to the same FoV as the telephoto shot, we have effectively changed the focal length of the lens, thus demonstrating the principle that different lenses on different format cameras can have the same effective focal length. I've always argued that it's the effective focal length that counts, not the lens per se.

(3) Whether the perspective distortion is caused by a change in focal length or a change in distance, it is a distortion in both cases, or an illusion if you like. Agreed?

(4) When I photograph a bird sitting on the branch of a tree from a distance of say 30 metres, using a telephoto lens, the image, or final print, really does give the impression the shot of the bird was taken from close up. If something in an image appears closer to the viewer than it actually was in reality, is that not perspective distortion? If not, what type of distortion or illusion would you call it?

(5) I'm sure we can both agree that the big nose effect from a wide-angle lens is a distortion or illusion. But what happens when the subject for the portrait really does have a huge nose in reality? Well, we can create the opposite effect by taking the portrait with a 300mm or even 600mm lens (35mm format equivalent) from a great distance. The nose might then appear normal, and that would be an illusion, although  probably a nice illusion.

Title: Re: Perspective Revisited
Post by: Ray on January 29, 2012, 02:12:40 am
Ray,

with apologies and please don't take any offense, but on this subject you really should leave it alone.  You just can't get past the idea that distortion is not perspective.  The point is when you remove the tube, you see more, but the perspective of the objects you saw with only the tube doesn't change.  The only way to change that is move your position in relation to those objects.  Make a framing guide from a piece of matt board and find a set of telephone poles receding in the distance.  Frame an image with the rectangle and note the perspective of the poles to each other.  When you remove the guide, that doesn't change.  You see more, and that's all focal length on a lens does.  Any other effect(distortion) is unintentional ( and now almost all of us correct it with software).

This insistence on this concept (you do realize that may be the ONLY one that believes this) reflects poorly on your credibility.



Wayne,
No offense taken, and you are probably right that I should leave it alone. The reason I don't is because I'm interested in clarity of thought and I sense a lot of confusion on the issue.

For example, if we address the points you've raised in your post, I sense the confusion continues, no offense intended.

You claim, for example, that distortion is not perspective, and that I can't get past this idea. I think I can. I can distinguish between perspective distortion and various types of lens distortions, such as barrel and pincushion distortion, and particularly 'volume anamorphosis' which is a real problem with ultra-wide-angle lenses near the edges and corners.

I maintain that after correcting for these distortions, the wide-angle lens still makes distant objects look significantly more distant. The perspective distortion I believe I see is not due to lens distortions, although such lens distortion may exacerbate the sense of perspective distortion if not corrected.

Quote
The point is when you remove the tube, you see more, but the perspective of the objects you saw with only the tube doesn't change.

As I've already pointed out, you get the same effect looking through a 50mm lens which matches the magnification of the eye (approximately).  Look at an object with camera raised to eye, with standard lens attached, then move the camera to one side, and look at the same object(s) with your naked eye. The objects in the scene appear the same size and therefore the same distance away.

Try the same experiment with a wide-angle lens and I'm sure you'll find that distant objects seem much more distant than they appear to the naked eye. The reverse will apply to the telephoto lens.

What perhaps I can't get past is the experience of what my eyes are telling me. A 50mm lens (or standard lens in relation to the format) provides close to the natural perspective that the naked eye sees, from whatever position.

Any change of effective focal length of lens, from the same position, produces a different picture with a sensation of a different perspective, in terms of perceived relative distances from the viewer to objects in the scene.

Now I admit this sense of different distance caused by 'zooming in' or 'zooming out', can be described as an illusion, but so can the different perspective of actually changing distance be described as an illusion, as in the big-nose-made-small example.


Title: Re: Perspective Revisited
Post by: Ray on January 30, 2012, 12:14:52 am
After giving this matter some more thought, I think now see the reason for this difference of opinion about perspective and focal length of lens.

Whenever I compare technical qualities in images, such as resolution, noise or DoF etc, I always compare equal size images or prints, because that's the sensible thing to do.

I've been assuming all along that that's what everyone else does, but it seems this isn't the case.

Guillermo's example of his microwave taken with a 10mm lens, showing the crop lines for 17mm and 22mm can only demonstrate that perspective doesn't change provided the smaller images resulting from the cropping are not enlarged, and providing all the different sized images or prints are viewed from the same distance, which they are in his example.

However, this effect is not what happens when one looks through the camera's viewfinder using different focal lengths of lenses. The viewfinder remains the same size, but the objects viewed become enlarged as a result of any increase in focal length of lens.

Likewise, when making prints to hang on one's wall, or to sell to customers, one would not choose to make the size of the print inversely proportional to focal lengthof lens used, in order to maintain the original perspective, although one could if one so chose.

I think most of us would agree that it would be a very odd thing to do, if a photographer of wildlife were to exhibit his prints at postage-stamp size on the grounds that he wanted to maintain the original perspective before raising camera to eye, with telephoto lens attached,  to take the shot.

Most of us want to exploit that potential of the telephoto lens to change perspective and make things look closer. We revel in the close-up view with its extra detail and hyper-realism.

Title: Re: Perspective Revisited
Post by: Fine_Art on January 30, 2012, 01:34:42 am
After giving this matter some more thought, I think now see the reason for this difference of opinion about perspective and focal length of lens.

Whenever I compare technical qualities in images, such as resolution, noise or DoF etc, I always compare equal size images or prints, because that's the sensible thing to do.

I've been assuming all along that that's what everyone else does, but it seems this isn't the case.

Guillermo's example of his microwave taken with a 10mm lens, showing the crop lines for 17mm and 22mm can only demonstrate that perspective doesn't change provided the smaller images resulting from the cropping are not enlarged, and providing all the different sized images or prints are viewed from the same distance, which they are in his example.

However, this effect is not what happens when one looks through the camera's viewfinder using different focal lengths of lenses. The viewfinder remains the same size, but the objects viewed become enlarged as a result of any increase in focal length of lens.

Likewise, when making prints to hang on one's wall, or to sell to customers, one would not choose to make the size of the print inversely proportional to focal lengthof lens used, in order to maintain the original perspective, although one could if one so chose.

I think most of us would agree that it would be a very odd thing to do, if a photographer of wildlife were to exhibit his prints at postage-stamp size on the grounds that he wanted to maintain the original perspective before raising camera to eye, with telephoto lens attached,  to take the shot.

Most of us want to exploit that potential of the telephoto lens to change perspective and make things look closer. We revel in the close-up view with its extra detail and hyper-realism.



Magnification, showing more detail is not perspective. Perspective is not angle of view either. Angle of view is related to magnification.

Wherever you are light is entering your eye in a straight line from everything it came from. If you move a few steps away the things that were behind other things may come into view. All the light is still entering the eye in straight lines. You could shine a laser pointer out to them.

With a telephoto lens all you are doing is magnifying the detail in a smaller arc. It does not change perspective. Everything still lines up to the same points when you scan the lens around. I understand what you are saying that the person looking at the shot may assume you were close to the animal based on their memory of how things look when they are there. That is valid, they may well make assumptions. It isnt perspective its angle of view. If they had the ability to notice how flat the image looked they would guess it was from far away. If it was a wide angle shot they could guess from the magnification distortion that it was a wide lens. They would not assume for example that buildings shrank for the photo.

The perspective of a viewpoint is what is in line with the eye. The relative size of those things is not the perspective, it is magnification distortion.
Title: Re: Perspective Revisited
Post by: Ray on January 30, 2012, 06:21:45 am
Magnification, showing more detail is not perspective. Perspective is not angle of view either. Angle of view is related to magnification.


Really! This is getting curiouser and curiouser. It seems pretty obvious to me that a change in the position of the viewer, whether the viewer is looking at 3-dimensional objects in a real scene or at a picture of that real scene hanging on the wall, will cause a change in angle between the line of sight from the eye to the proportions of the objects being viewed.

For example, if I view a tall building from such a distance that it looks very small, a mere detail in the landscape, the angle between the rays of light from the top of the building to my eyes, and the rays of light from the bottom of the building to my eyes, will be very narrow indeed. Agreed?

If I move up really close to the building, which will cause a change in perspective (I've never denied that. I'm not silly), then the angle between those rays reflecting from the top and bottom of the building to my eyeballs will be very wide indeed. Agreed!

How can you claim that perspective is not related to angle of view?

Please give an example of a change of perspective which does not involve a change in angle of view.
Title: Re: Perspective Revisited
Post by: Eric Myrvaagnes on January 30, 2012, 07:43:18 am
Please give an example of a change of perspective which does not involve a change in angle of view.

Example:

Find two trees or other large objects that are a good 100 feet or more apart. Take a camera with your favorite fixed-focal length lens with you and go to a position where you are at least 500 feet from the nearer tree and where the more distant tree is still visible (that is, you are a little ways off from a straight line joining the trees, so you can see both of them. Take a picture of the two trees with a small lens opening and focused on the nearer tree.

Then, walk toward the nearest tree until you are about 20 feet from it, with the more distant tree still showing behind it. Don't forget to bring your camera and the same lens with you. Without changing the focus or lens opening from the first picture, take another picture of the two trees.

Since you used the same lens on the same camera, the viewing angle is identical in both images. But the perspective (as well as your invention of the "perception of perspective") will have changed.

Isn't this obvious? Please note that distortion will not have any different effect in the two images, since you used the same camera, lens, focus, and aperture for both pictures. Of course, the sun may have moved a little, or even gone behind a cloud between photos 1 and 2, but I hope even you won't consider exposure to be related to perspective.

Eric
Title: Re: Perspective Revisited
Post by: Ray on January 30, 2012, 09:53:42 am
Example:

Find two trees or other large objects that are a good 100 feet or more apart. Take a camera with your favorite fixed-focal length lens with you and go to a position where you are at least 500 feet from the nearer tree and where the more distant tree is still visible (that is, you are a little ways off from a straight line joining the trees, so you can see both of them. Take a picture of the two trees with a small lens opening and focused on the nearer tree.

Then, walk toward the nearest tree until you are about 20 feet from it, with the more distant tree still showing behind it. Don't forget to bring your camera and the same lens with you. Without changing the focus or lens opening from the first picture, take another picture of the two trees.

Since you used the same lens on the same camera, the viewing angle is identical in both images. But the perspective (as well as your invention of the "perception of perspective") will have changed.

Isn't this obvious? Please note that distortion will not have any different effect in the two images, since you used the same camera, lens, focus, and aperture for both pictures. Of course, the sun may have moved a little, or even gone behind a cloud between photos 1 and 2, but I hope even you won't consider exposure to be related to perspective.

Eric

Eric, We both know I can walk anywhere I like, jump up and down, have dinner and change my clothes, but neither the viewing angle of my eyes nor the viewing angle of my lens will change, unless I attach another lens to the camera and /or change the magnification of my spectacles.

The angles that change are the angles that delineate the objects in the scene from the perspective of the eye or the camera lens/sensor. These angles will change if one changes position in relation to any fixed object in the scene.

The point I've been addressing all along, is not a denial that changing position changes perspective, but that there seem to be additional factors such as focal length of lens which contribute to a change of perspective, whether from the same position or a changed position.

As I'm sure you know, in order to demonstrate a point scientifically one should try to eliminate as many variables as possible.

In this digital age we experience image size in relation to pixel count. Irrespective of the size of the sensor, images of the same aspect ratio and pixel count will appear the same size at 100% on screen, or make the same size prints at the same ppi, without downsampling or interpolation.

So, if we're trying to demonstrate, like Guillermo has attempted, that focal length has no bearing on perspective, how about eliminating the variable of pixel count for a start, and use the same camera and same sensor with different focal lengths of lenses resulting in the same size image or print.

We could exagerate a little to get the point across. We could use a 12mm full-frame lens that captured almost the whole of the kitchen with Guillermo's microwave oven looking quite distant. Then we could use, say, a 100mm lens to fill most of the frame with just the microwave oven, both shots taken from the same position.
We could then display both images at the same physical size, say 24"x16" prints next to each other on the same wall, view them from the same distance, and ponder whether or not the perspective in both images is the same.

We know that the photos were taken from the same position. We know that the prints are viewed from the same position. We know that the camera and sensor are the same. The only variable is the focal length of lens.

That's the way to do it.  ;D
Title: Re: Perspective Revisited
Post by: JohnTodd on January 30, 2012, 01:13:28 pm
Ray,

Would it be possible for you to upload a couple of pictures which demonstrate the effect you're talking about, so we can understand it by comparison?
Title: Re: Perspective Revisited
Post by: Guillermo Luijk on January 30, 2012, 02:35:32 pm
I answer over your text:

So, my question to you, Guillermo, is do you agree with the following statements, and if not why not?

(1) Without the rest of the face visible, the size of the nose in relation to the eyes and ears, cannot be gauged in the telephoto shot. We cannot determine whether or not the nose is unusually large or not.

Correct. Both images would be undistinguishable.

(2) If we crop the wide-angle shot to the same FoV as the telephoto shot, we have effectively changed the focal length of the lens, thus demonstrating the principle that different lenses on different format cameras can have the same effective focal length. I've always argued that it's the effective focal length that counts, not the lens per se.

Incorrect. The focal length of a lens is a physical optical parameter, measured in mm. By cropping you only change the FOV. Different lenses on different formats can have the same FOV, not the same focal length. The term 'effective focal length' is incorrect, there is not such thing. There is just a FOV produced by the combination of a given focal length + sensor format. The term 'equivalent focal length' could be acceptable IMO, even if it doesn't refer to a focal length but to the FOV the lens would produce in a 35mm format.

(3) Whether the perspective distortion is caused by a change in focal length or a change in distance, it is a distortion in both cases, or an illusion if you like. Agreed?

The combination of perspective (which solely depends on distance) plus FOV (determined by the combination of focal length + sensor format) can make use perceive distorted subjects. The reason for this is basically because our visual system doesn't project images linearly nor has such a wide FOV as a wide angle lens can produce. This is why wide angle images look weird to us many times, because our eyes never produce them in real life. A wide angle image showing distortion is not incorrect, it's just a mathematical projection our visual system cannot produce by itself. Is this an illusion? I would rather consider it a mismatch between the image produced in the camera vs the images we are used to observe in real life.

(4) When I photograph a bird sitting on the branch of a tree from a distance of say 30 metres, using a telephoto lens, the image, or final print, really does give the impression the shot of the bird was taken from close up. If something in an image appears closer to the viewer than it actually was in reality, is that not perspective distortion? If not, what type of distortion or illusion would you call it?

Incorrect. It is not perspective, it is magnification, and as Fine_Art explained well this is related to FOV.

(5) I'm sure we can both agree that the big nose effect from a wide-angle lens is a distortion or illusion. But what happens when the subject for the portrait really does have a huge nose in reality? Well, we can create the opposite effect by taking the portrait with a 300mm or even 600mm lens (35mm format equivalent) from a great distance. The nose might then appear normal, and that would be an illusion, although  probably a nice illusion.

Correct. Perspective (i.e. subject distance) can be used to fake the perception of nose's size in a portrait. You can take a picture of a big nosed guy from a long distance, and make it look a smaller nose than a wide angle shot taken from a very short distance from a guy with a regular nose.



Title: Re: Perspective Revisited
Post by: Eric Myrvaagnes on January 30, 2012, 03:08:19 pm
Ray,

Would it be possible for you to upload a couple of pictures which demonstrate the effect you're talking about, so we can understand it by comparison?
Excellent request. I tried to answer quite explicitly the exact question that Ray asked, namely "Please give an example of a change of perspective which does not involve a change in angle of view."

Photographing any scene with any one camera and the same fixed lens at different distances gives "a change of perspective which does not involve a change in angle of view" because the angle of view of any fixed lens is fixed (i.e, it doesn't change; that's why it's called 'fixed.')

But Ray's answer to me goes into completely unrelated territory so I have no idea what relation, if any, his comments have to "perspective."

Eric

P.S. Guillermo, on the other hand, makes complete sense.
Title: Re: Perspective Revisited
Post by: dchew on January 30, 2012, 04:47:36 pm
Ray,
I think you have a very interesting point of view on this subject.  However, I keep coming back to what others have stated: this is all about the definition.  I have always thought of perspective as the relative size of things I see (sorry - imprecise layman's terms).  In photography the definition of perspective is "usually" limited to relative size and how it changes as you move closer to or farther away from subjects.  Our vision is of course no different than a camera in this regard.  If I stand 6" from a tree trunk it will encompass almost my entire view, and appear much bigger than another tree trunk 50 feet away.  But If I stand 50 feet away from the first tree trunk, the second one (now 100 feet away) will appear much closer in size to the front trunk.  Our brains know this and adjust for it but the camera doesn't.  This effect is what I call perspective.

You want to include other things in that definition of perspective.  That's fine if you want to do that.  The reason I don't want to do it is because it might confuse the definition of some of those other things.  For example, someone may ask why the bushes at the bottom corner of a wide angle shot are stretched and leaning?  Or why is the building tipping back? Or why is the dog's nose so big relative to his tail? Or why does the telephoto image look sharper vs. the center crop of the wide angle blown up to the same size?

For me it makes sense to relate each one of those questions/answers to a distinct (and different) cause/effect.  I get the sense you want to include some additional cause/effect into "perspective."  Again, fine.  But that's a debate about the definition of perspective.  And if we can't agree on the definition of something, then we obviously can't agree on what that "something" is or does.

Dave
Title: Re: Perspective Revisited
Post by: Wayne Fox on January 30, 2012, 07:50:03 pm
Ray,
I think you have a very interesting point of view on this subject.  However, I keep coming back to what others have stated: this is all about the definition.  ...

Dave
This has been the issue with Ray and perspective all along in the other threads.  He wants to roll other things into the concept of "perspective" and redefine it, and while I suppose if  you go by the broad definition of perspective it might be understandable, the term is very well defined and accepted in visual arts.  Trying to explain other phenomenon as part of perspective goes against the accepted definition, as this thread as so aptly pointed out.  Trying to roll FoV, distortion and perspective into one catchall concept of perspective seems illogical, especially since all three and their relationships are pretty well defined and understood by photographers  (and others involved in visual arts), and understanding them separately is important and beneficial whereas lumping them together doesn't accomplish anything.

Ray has a different perspective on perspective (sorry, couldn't help myself), and despite all of his attempts to convince others, nothing has changed.
Title: Re: Perspective Revisited
Post by: Eric Myrvaagnes on January 30, 2012, 09:16:01 pm
While we're turning "perspective" into a universal catch-all concept, why don't we also throw in exposure, dynamic range, the ratio of curent value of the US dallar against the Canadian dollar, what I had for supper last night, and the number of colors in the third shirt that Jeff will be wearing in the LR4 video when it comes out? And, while we're at it, why not...

 ;)

I guess it's time to go take some pictures, folks.

Eric  ;D
Title: Re: Perspective Revisited
Post by: Fine_Art on January 31, 2012, 12:08:39 am
Really! This is getting curiouser and curiouser. It seems pretty obvious to me that a change in the position of the viewer, whether the viewer is looking at 3-dimensional objects in a real scene or at a picture of that real scene hanging on the wall, will cause a change in angle between the line of sight from the eye to the proportions of the objects being viewed.

For example, if I view a tall building from such a distance that it looks very small, a mere detail in the landscape, the angle between the rays of light from the top of the building to my eyes, and the rays of light from the bottom of the building to my eyes, will be very narrow indeed. Agreed?

If I move up really close to the building, which will cause a change in perspective (I've never denied that. I'm not silly), then the angle between those rays reflecting from the top and bottom of the building to my eyeballs will be very wide indeed. Agreed!

How can you claim that perspective is not related to angle of view?

Please give an example of a change of perspective which does not involve a change in angle of view.


You are taking a picture of a famous sculpture or building that is down the end of the street. eg (http://msnbcmedia1.msn.com/j/ap/2425cbcc-0ad1-45db-8405-4ebbece69448.grid-4x2.jpg)
http://www.oldnorth.com/history/index.htm (http://www.oldnorth.com/history/index.htm)

You take a picture from the sidewalk. It looks a bit cramped. You check for cars then move your tripod at right angles to take the shot from the middle of the street. You have changed your perspective. Your angle of view has not changed. The building is still the same size in relation to the other buildings.
Title: Re: Perspective Revisited
Post by: Ray on February 02, 2012, 04:39:58 am
I answer over your text:

Very brave!  ;D

2) If we crop the wide-angle shot to the same FoV as the telephoto shot, we have effectively changed the focal length of the lens, thus demonstrating the principle that different lenses on different format cameras can have the same effective focal length. I've always argued that it's the effective focal length that counts, not the lens per se.

Quote
Incorrect. The focal length of a lens is a physical optical parameter, measured in mm. By cropping you only change the FOV. Different lenses on different formats can have the same FOV, not the same focal length. The term 'effective focal length' is incorrect, there is not such thing. There is just a FOV produced by the combination of a given focal length + sensor format. The term 'equivalent focal length' could be acceptable IMO, even if it doesn't refer to a focal length but to the FOV the lens would produce in a 35mm format.

Guillermo, you will find that the terms "effectively the same" and "equivalent" are used interchangeably in the English language when referring to similarities in the focal length of lenses and other matters. The terms are synonymous. "Effectively the same" means "equivalent". If you disagree, I would describe that as a semantic quibble.

Here's one definition of equivalence from the Encarta Dictionary.

Quote
equivalence | equivalency:
The fact of being the same, effectively the same, or interchangeable with something else.

It is understood that two lenses of different 'actual' focal length, but equal 'effective' or 'equivalent' focal length as a result of cropping, are not necessarily equivalent in other respects, such as maximum F/stop, nearest focussing distance, vignetting, MTF response and a whole lot of distortions such as barrel distortion and volume anamorphosis.

It should also be understood, when attempting to compare the perspective in shots taken from the same position with different 'equivalent' focal lengths of lens, one should remove as far as possible such lens distortions from each shot in order not to prejudice the findings. The lens modules in ACR in CS5 do a pretty good job of removing or reducing many distortions, but don't appear to have any effect on volume anamorphosis. To fix that, one needs the sort of converter and lens modules provided by DXO Labs, or do it manually with Photoshop's free transform and warp.

Having successfully removed such distortions before the comparison of the perspective issue, one cannot prove that perspective is not affected by differences in 'equivalent' focal length, or FoV, by making the equivalent focal length the same in both images through cropping.

That would almost be like claiming that 2 = 5, and to  prove it I'll remove 3 from the 5. So, whilst it's not true that 2 = 5, it is true that 2 = 5 - 3.

The analogy here is that 2 = the shot with the long lens, and 5 = the shot with the shorter lens. The perspective in 2 is not equal to the perspective in 5, but it is equal if you subtract all the additional objects in the wider field of 5.

Your demonstration that FoV does not affect perspective, using the 10mm shot of your microwave oven, with overlay crop marks for 17mm and 22mm equivalence, is merely a tautology. You are basically stating the obvious that 2 (22mm) = 5 (10mm) minus 3 (cropping).

Let's consider again the example of the person with the big nose. I take a portrait with a standard lens that is not flattering but is accurately realistic because the big nose is very apparent. I decide to take another shot from a much greater distance which is more flattering, but I have only the one standard lens. However, my camera is an IQ180 with 60mp, and I reckon with the high quality pixels of the IQ180 I can make a reasonable A3 portrait from a crop consisting of 2 or 3 mp.

Now you are claiming that it makes no difference to the perspective distortion of the nose whether I use a 400mm lens or an 80mm lens to capture the head and shoulders portrait, whereas I'm suggesting that without the cropping and magnification of the 80mm shot, the perspective will not look distorted.

In other words, if the field size (how much of the subject and its surrounding area is visible) is different, then the sense of perspective in the image from the viewpoint of the viewer will be different. The perspective distortion will not be apparent when the subject is surrounded by similarly distorted and distant background objects.

If one crops a distant object, divorces it from its surroundings then enlarges it, the perspective that was seen as being natural for that object when viewed at a distance in a photo taken with a standard lens, will then become unnatural, although more flattering in our example.

There is also an obvious contradiction in your methodology of examing perspective based on identical distance from subject to viewer. In order to determine that the perspective of distant small objects in a narrow field cropped from a wide-angle shot is the same as another shot with a longer lens that encommpasses the same narrow field, you have to break the condition of equal distance. You have to get closer to the small crop to peer at it with a magnifying glass.

In other words, you are 'effectively' saying, "The perspective in shots taken with different 'actual' and different 'equivalent' focal lengths of lenses, from the same position, can be demonstrated as being the same, provided I convert both images to the same 'equivalent' focal length, and provided I change my viewing distance to the smaller crop from the wider-angle shot.

What a balls-up! What sort of a scientific method is this!  ;D

I'll repeat. The proper way to examine this issue is to eliminate all variables as far as possible. Keep the distance from the scene the same in all shots. Keep the camera the same and the print sizes the same. Don't 'doctor' the evidence by cropping in post-processing, which is tantamount to 'selection bias'. Don't make different size prints of the images being compared, and don't view such prints from different distances.

If you follow this sound scientific procedure, I think you will find that FoV does indeed influence the sense of perspective in a scene, in the mind of the viewer.

However, divorced from the viewer, the perspective does not change. But that's Alice in Wonderland stuff.  ;D
Title: Re: Perspective Revisited
Post by: Ray on February 02, 2012, 05:04:46 am
You take a picture from the sidewalk. It looks a bit cramped. You check for cars then move your tripod at right angles to take the shot from the middle of the street. You have changed your perspective. Your angle of view has not changed. The building is still the same size in relation to the other buildings.

Of course your angle of view has changed. Any change in position, up or down, to one side or another, forwards or backwards, is a change in perspective and angle of view.

If it's a small change in position, the change in perspective and angle of view will probably be small. If it's a large change in position, the change in perspective will probably be large.

Changes in position, even though minor, can cause objects that were previously obscured to become visible, and objects that were previously visible to become obscured, because the angle of view has changed.

Changes in FoV can cause huge numbers of objects that were previously obscured to become visible.

Perhaps I should clarify what I mean by angle of view. I'm making a distinction between 'angle of view' and 'field of view'. The FoV of the lens/camera doesn't change as you change position, obviously. How can it? That's a property of the lens/camera system. What changes is the visual angle from the eye to the various objects within the field.
Title: Re: Perspective Revisited
Post by: JohnTodd on February 02, 2012, 08:12:55 pm
Ray,

Are you talking about the difference between the two attached images (when seen opened at full size)? To clarify, the pixel relationship between the toy dog and the distant building is exactly the same, the only difference is the presence or absence of surrounding detail.

John
Title: Re: Perspective Revisited
Post by: Ray on February 03, 2012, 05:57:13 am
Ray,

Are you talking about the difference between the two attached images (when seen opened at full size)? To clarify, the pixel relationship between the toy dog and the distant building is exactly the same, the only difference is the presence or absence of surrounding detail.

John


John,
The surrounding detail from top to bottom is exactly the same. All you seem to have done is change the aspect ratio of the image by cropping the sides.

During these discussions in other threads, it was mentioned that merely pointing the camera in a slightly different direction whilst standing in the same position, constitutes a change in position. One might quibble again over the semantics. Is a change in the angle a lens is pointing a change in position? One might more accurately describe it as a change in angle from the same position. However the effect of a change in angle can be quite dramatic. Merely pointing the camera slightly upwards or downwards can have a substantial effect on the verticals of buildings.

If you had taken those two shots, as presented, with different focal lengths of lens, you would have had to have either pointed the telephoto lens down slightly, or crouched down to a lower position. You would have needed to change position and you would have lost the top part of the image. As we all know, a change in position results in a change of perspective.

Now I'm travelling at the moment in Thailand. However, because I'm retired, a holiday can be as long as I want  it to be. I have no compulsion to rush around to see everything, and cram every sight-seeing opportunity into the available time. I can relax and continue many normal activities such as posting on LL. However, processing images on my notebook can be a bit of a chore. I'm reluctctant to bother because the results in terms of color calibration can be off.

Nevertheless, for the record I probably should photograph the view from the top floor of my current hotel. Even though it's not spectacular, it's fairly nice.

So here are the two shots, taken earlier today, using the same camera from the same position and angle, displayed at the same size, and presumably viewed from the same distance. The only significant difference is the focal length of the lenses used.

Title: Re: Perspective Revisited
Post by: Eric Myrvaagnes on February 03, 2012, 09:33:26 am
So here are the two shots, taken earlier today, using the same camera from the same position and angle, displayed at the same size, and presumably viewed from the same distance. The only significant difference is the focal length of the lenses used.


And the "perspective" in both pictures is the same, although the difference in "field of view" makes things look closer in one than the other. Is this the effect you have recently been calling "perception of perspective?" The "effect of field of view" might be a better term, but if you want to define a term in a non-standard way, you would help your cause if you provided a clear definition of it.
Title: Re: Perspective Revisited
Post by: Guillermo Luijk on February 03, 2012, 09:43:34 am
when referring to similarities in the focal length of lenses and other matters. The terms are synonymous. "Effectively the same" means "equivalent". If you disagree, I would describe that as a semantic quibble.transform and warp.

Your entire discussion about perspective IS about semantics.

But your particular view of what 'focal length' means is not a semantic discussion, you are simply wrong in this concept. There is no such thing as 'equivalent focal length' or whatever (no matter how many badly informed photographers use that term), because the focal length of a lens is UNIQUE. It is a physical parameter of the optics and is measured in mm. What you call 'equivalent focal length' refers to the field of view (FOV), and that's the proper word for it. And the FOV depends on the pair: focal length + format size (this including any pp cropping).

A 50mm is always a 50mm, no matter in which sensor size you put it. It's a 50mm on a FF camera, and it's a 50mm on an APS-C camera, but will provide a different FOV on both. A 50mm will provide on a FF body the same FOV as a 33mm on an APS-C camera, and a 50mm will provide on an APS-C camera the same FOV as a 75mm on a FF camera. As simple as this.

Regards
Title: Re: Perspective Revisited
Post by: Eric Myrvaagnes on February 03, 2012, 05:52:30 pm
Your entire discussion about perspective IS about semantics.
Yes! Absolutely!

And it's worth repeating:
Your entire discussion about perspective IS about semantics.

And again, just to be certain:
Your entire discussion about perspective IS about semantics.

Eric
Title: Re: Perspective Revisited
Post by: feppe on February 03, 2012, 06:24:26 pm
I can't believe the 2nd edition of this debate is going on for another four pages.

What is the authoritative scholarly source (read: a book) which someone could refer to settle this? I'm positive these things have been put on paper hundred plus years ago. Having such drawn out and controversial threads on some of the most basic subjects of optics on a supposedly pro photography and pixel peeper forum is truly incomprehensible, and frankly embarrassing.
Title: Re: Perspective Revisited
Post by: Wayne Fox on February 03, 2012, 11:33:22 pm
What is the authoritative scholarly source (read: a book) which someone could refer to settle this? I'm positive these things have been put on paper hundred plus years ago. Having such drawn out and controversial threads on some of the most basic subjects of optics on a supposedly pro photography and pixel peeper forum is truly incomprehensible, and frankly embarrassing.
A little embarrassing (for ray anyway). I guess embarrassing for others because keep bashing our heads against it and keep the thing alive.  I'm not sure about "scholarly" source, but there are probably 100's of places that talk about "perspective" as it relates to photography (including threads like this which contributors have made it pretty clear), and ray is the only person I've ever seen try to "redefine" the accepted understanding of the photographic world (and other visual arts for that matter).

It's semantics for sure, problem is their is an accepted meaning, ray just doesn't want to believe it because he wraps his own personal thoughts around it.

I's obviously pointless, even though not a single person as chimed in to "defend" his personal definition or decided he is right, he just doesn't seem to understand that while there might be some "logic" to his thought process, there is an accepted concept of perspective in photography (and other visual arts), that is valuable as it is, easy to teach and important to be separate from some of the other concepts he is trying to combine it with.
Title: Re: Perspective Revisited
Post by: ErikKaffehr on February 04, 2012, 01:01:02 am
Hi,

I started working on an article on the issue, there are some pictures here:http://echophoto.dnsalias.net/ekr/index.php/photoarticles/64-lnses-in-perspective

Later I will also add a few samples demonstrating projection.

Best regards
Erik


And the "perspective" in both pictures is the same, although the difference in "field of view" makes things look closer in one than the other. Is this the effect you have recently been calling "perception of perspective?" The "effect of field of view" might be a better term, but if you want to define a term in a non-standard way, you would help your cause if you provided a clear definition of it.
Title: Re: Perspective Revisited
Post by: Ray on February 04, 2012, 03:47:10 am
And the "perspective" in both pictures is the same, although the difference in "field of view" makes things look closer in one than the other. Is this the effect you have recently been calling "perception of perspective?" The "effect of field of view" might be a better term, but if you want to define a term in a non-standard way, you would help your cause if you provided a clear definition of it.

Eric,
The definition I'm using is the one that the Department of Media Studies at Chicago University finds the most useful, taken from the Oxford English Dictionary, that ultimate authority on the meaning and use of words and terms in the English language.

Here's an extract from their article on perspective at  http://csmt.uchicago.edu/glossary2004/perspective.htm

Quote
The most useful definition of perspective for media studies found in the Oxford English Dictionary is, “The art of drawing solid objects on a plane surface so as to give the same impression of relative position, size, or distance, as the actual objects do when viewed from a particular point.”

You'll notice that there are two key words in this definition that appear to be missing from your definition (implied in your comments), namely, impression and distance.

Perspective provides an impression of the relative size and distance of objects as seen by the naked eye, or some imaginary eye, placed at a particular position or viewpoint.

Now you have claimed that the perspective in both the 24mm shot and the 120mm shot is the same, but also admit that the 120mm lens has made things look closer. How can that be? Can't you see the contradiction?

What I see in these two images, is that the 24mm shot has caused distant objects to appear slightly further away than they would actually appear with the naked eye from the same position. I'd describe it as a distortion of perspective due to the FoV, in conjunction with the effects of magnification and/or reduction. However, what seems undeniable to me is that the sense of perspective in the two images is different, to the viewer.

In the 120mm shot I see a slight, but clearly noticeable, compression of the buildings and objects in the field, which also represents a slight distortion of perspective. The image creates the impression that the viewer is much closer to all objects in the field, but the distances between the separate objects, from the nearest object to the furthest, as judged by their relative size, is a little unnatural and to some degree in conflict with the impression that all objects are much closer to the viewer.

What I see in these two images are two types of perspective distortion, namely, extension distortion and compression distortion. To claim that perspective is the same in both images when it is clear that in one image we have perspective extension distortion, and in the other image, perspective compression distortion, seems a little crazy to me, Eric.

Is Alice in Wonderland you're favourite book?  ;D

Title: Re: Perspective Revisited
Post by: Ray on February 04, 2012, 03:55:59 am
Your entire discussion about perspective IS about semantics.

But your particular view of what 'focal length' means is not a semantic discussion, you are simply wrong in this concept. There is no such thing as 'equivalent focal length' or whatever (no matter how many badly informed photographers use that term), because the focal length of a lens is UNIQUE. It is a physical parameter of the optics and is measured in mm. What you call 'equivalent focal length' refers to the field of view (FOV), and that's the proper word for it. And the FOV depends on the pair: focal length + format size (this including any pp cropping).

A 50mm is always a 50mm, no matter in which sensor size you put it. It's a 50mm on a FF camera, and it's a 50mm on an APS-C camera, but will provide a different FOV on both. A 50mm will provide on a FF body the same FOV as a 33mm on an APS-C camera, and a 50mm will provide on an APS-C camera the same FOV as a 75mm on a FF camera. As simple as this.

Regards


But I do understand everything you've written above, Guillermo, just as a I understand that 2 = 2.

If I didn't know that a 50mm lens is a 50mm lens, I'd have great trouble selecting the appropriate lens to use or take with me when I go out shooting. I might take a 90mm lens instead of a 50mm lens, then wonder why I wasn't getting the expected FoV.

I hope I never get to that stage of senility  ;D . I also realise that a camera is a camera and not a pencil-sharpener, for example.

Quote
What you call 'equivalent focal length' refers to the field of view (FOV), and that's the proper word for it.

I see! So you consider the term 'equivalent focal length' improper. You think people might get confused by it? Or perhaps you think it's just poor English and logically incorrect.

From my perspective, to describe two lenses as having equivalent "Fields-of-View" seems a bit crazy. A field is not a property of a lens. It's an external view which one may hope to capture, and which will vary precisely according to one's position and location, wherever one might be, whatever the lens one uses. There is no equivalent Field-of-view, only Fields-of-View which are either the same or different.

However, a Focal Length is clearly a property of the lens, and an equivalent Focal Length does not change whatever one's position. Equivalent Focal length means, 'whatever the view, whatever the field, whatever one's position in whatever location, the lens will capture the same field as another lens of equivalent focal length.'

'Equivalent focal length' is a much better expression in my view, but 'effectively the same focal length' is also meaningful.


Title: Re: Perspective Revisited
Post by: ErikKaffehr on February 04, 2012, 04:17:07 am
Hi,

I have shot some illustrations on the issue:

Sample 1: Three images taken from the same tripod position with three lenses and cropped to same size.

Sample 2: The same three images, uncropped.

Full article here: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/64-lnses-in-perspective

Best regards
Erik

Title: Re: Perspective Revisited
Post by: Guillermo Luijk on February 04, 2012, 06:36:10 am
I see! So you consider the term 'equivalent focal length' improper. You think people might get confused by it? Or perhaps you think it's just poor English and logically incorrect.

It is improper, inaccurate and injustified*. For someone who knows about photography it is easy to understand 'this has an equivalent focal length of 85mm', meaning that when you put a 50mm on an APS-C sensor you get the same FOV as an 85mm would provide on a FF body. But I am tired of reading misleaded new users in the forums thinking the focal length of their lens is going to change according to the camera they set it.

* The days in which the 35mm format was a universal reference passed away. Today 99% of digital cameras in the world are not FF and there are many more APS-C sized cameras than FF for instance.
Title: Re: Perspective Revisited
Post by: Bart_van_der_Wolf on February 04, 2012, 06:43:21 am
I see! So you consider the term 'equivalent focal length' improper. You think people might get confused by it? Or perhaps you think it's just poor English and logically incorrect.

All of the above. It's supposed to be an abbreviation of; "a different focal length that produces an equivalent Field of View on a sensor with a different size", but causes a lot of confusion instead.

Cheers,
Bart
Title: Re: Perspective Revisited
Post by: ErikKaffehr on February 04, 2012, 07:05:14 am
Oh now! Guillermo you don't understand this! All APS-C sensors have a built in 1.5X extender, I tought you knew!

Best regards
Erik
Title: Re: Perspective Revisited
Post by: Ray on February 04, 2012, 07:16:30 am
I'll just add that the wide-angle shots in Erik's presentation show a building which is clearly more distant than in the cropped versions.

I've tried to point out many times that cropping is the same as creating a different 'equivalent' focal length of lens. Two images taken from the same position, which are cropped to the same angle-of-view, are images taken with the same 'effective' focal length of lens, and that's what counts regarding perspective.

However, it is also true that the lens per se does not necessarily have any bearing on perspective. The expression 'per se' means 'in itself', ie. divorced from other considerations.

Guillermo has tried evade this obvious reality by changing the definition of 'equivalent focal length' to equivalent Field-of-View. What on earth is an equivalent 'field of view'?  Do we have to pretend that obviously different fields, containing different objects, are the same?

I can use both equivalent and non-equivalent focal lengths of lens to provide different 'Fields-of-View' simply by moving backwards or forwards. The perspective will then be slightly different of course as a result of the change of position, as well as the change in the field of view.

I can use lenses of equivalent focal length, and no matter what position I take, the equivalent focal length of the lens will not change, but the field of view certainly can change. It's different for every scene.

Some of you guys are certainly confused  ;D .
Title: Re: Perspective Revisited
Post by: Fine_Art on February 04, 2012, 08:02:43 pm
Of course your angle of view has changed. Any change in position, up or down, to one side or another, forwards or backwards, is a change in perspective and angle of view.

If it's a small change in position, the change in perspective and angle of view will probably be small. If it's a large change in position, the change in perspective will probably be large.

Changes in position, even though minor, can cause objects that were previously obscured to become visible, and objects that were previously visible to become obscured, because the angle of view has changed.

Changes in FoV can cause huge numbers of objects that were previously obscured to become visible.

Perhaps I should clarify what I mean by angle of view. I'm making a distinction between 'angle of view' and 'field of view'. The FoV of the lens/camera doesn't change as you change position, obviously. How can it? That's a property of the lens/camera system. What changes is the visual angle from the eye to the various objects within the field.


What you mean by angle of view doesn't matter. It is a mathematical definition.

"
For a lens projecting a rectilinear image, the angle of view (α) can be calculated from the chosen dimension (d), and effective focal length (f) as follows:[3]
(http://upload.wikimedia.org/wikipedia/en/math/c/9/6/c9652413291248e3eb7413571cadb79c.png)

d represents the size of the film (or sensor) in the direction measured. For example, for film that is 36 mm wide, d = 36 mm would be used to obtain the horizontal angle of view.
(http://upload.wikimedia.org/wikipedia/en/thumb/7/72/Angle_of_view.svg/260px-Angle_of_view.svg.png)
Because this is a trigonometric function, the angle of view does not vary quite linearly with the reciprocal of the focal length. However, except for wide-angle lenses, it is reasonable to approximate  radians or  degrees.
http://en.wikipedia.org/wiki/Angle_of_view (http://en.wikipedia.org/wiki/Angle_of_view)

This is not 'the world according to Ray'. :)

When painters say they draw with perspective it is drawing to try to give an impression of depth based on distance magnification. It is done with a "vanishing point" implying infinity distance and lines out from that point radially. The faster the lines diverge the faster the image seems to recede.

The perspective is from the position of the eye in 3 dimensions. The lines from that point go out to hit objects in the field of view you choose with a lens. The light comes in from those objects. If you move sideways to the objects some things may be obscured by others. The angles of lines from the camera have not changed. All that has changed is what is in front.
Title: Re: Perspective Revisited
Post by: j-land on February 04, 2012, 08:19:09 pm
I'll just add that the wide-angle shots in Erik's presentation show a building which is clearly more distant than in the cropped versions.


"Apparently more distant", NOT "clearly more distant", because in fact the building is exactly the same distance from the camera in all shots. When you look at things through binoculars they are apparently closer because the binoculars create a more magnified image compared to your eyes alone. Having said that, however, I wouldn't even say that it is clear that the building is apparently more distant in the wide angle image or apparently closer in the telephoto image. Because I'm familiar with looking at photographs and the properties of lenses, the relative size of things in a telephoto image (what's called "compression") gives me a clue that the things in the photograph were actually quite distant from the camera. So the spacial perception of the photograph can vary from viewer to viewer - which is anything but clear ;)
Title: Re: Perspective Revisited
Post by: Wayne Fox on February 04, 2012, 08:46:57 pm
"Apparently more distant", NOT "clearly more distant", because in fact the building is exactly the same distance from the camera in all shots. When you look at things through binoculars they are apparently closer because the binoculars create a more magnified image compared to your eyes alone. However I would say that it is clear that the building is apparently more distant in the wide angle image.  ;)
yes, and ray doesn't get it that just because you can see more of the scene in a wide angle shot the perspective hasn't change. While in a very loose sense the actual definition of the word perspective might apply, regarding the accepted use of the term in photography the perspective hasn't changed - photographers don't use the term perspective to describe a property of the actual image/print but more as a property important to understand and take advantage of when capturing images.  (granted many don't understand this point, so some of the confusion as often we see photographers recommending to switch to a telephoto or wide angle to change the perspective.  Implied in that comment is also repositioning yourself to change the size relationships of the object in the scene, but most don't elaborate on that point). Once you have taken a shot, you cannot change it's perspective by simply cropping it different ... which is more about FoV.  As I've mentioned before, ray is trying to merge at least 3 concepts perspective, distortion, and FoV as we use them in photography (and are very useful being separate) into a single concept he calls perspective.

Honestly I'm amazed he doesn't get it, and keeps defending it even though he may be the only person who believes it.

There is only one way to change perspective, and that is to change your point of view when taking an image (position relative to the objects you are photographing).  Sticking on a wide angle lens changes FoV, and may add distortion, but has no effect on perspective. Perspective has nothing to do with how much of a scene is revealed in the final image/print.

At least this thread is good for a nice chuckle each day ...
Title: Re: Perspective Revisited
Post by: Ray on February 04, 2012, 09:11:22 pm
It is improper, inaccurate and injustified*. For someone who knows about photography it is easy to understand 'this has an equivalent focal length of 85mm', meaning that when you put a 50mm on an APS-C sensor you get the same FOV as an 85mm would provide on a FF body. But I am tired of reading misleaded new users in the forums thinking the focal length of their lens is going to change according to the camera they set it.

* The days in which the 35mm format was a universal reference passed away. Today 99% of digital cameras in the world are not FF and there are many more APS-C sized cameras than FF for instance.


Guillermo,
I've never met anyone who thinks a lens will change its actual  physical and material properties when attached to a different camera body, but I guess someone who believes in magic might think that.

I doubt that referring to lenses as having the equivalent FoV would be less confusing for such people, because the FoV is not marked on a lens. I doubt whether you would always find a reference to it in the manual that comes with a lens.

How are such people going to describe the FoV? In terms of angles? Perhaps the confused people you are addressing with your proper definition might find it useful to mark on their lenses the FoV, horizontally and vertically, so they know which lens to grab for a particular shot.

I suspect even many who are experienced photographers don't actually know the FoV of their lenses and don't think in terms of angles. "Hhmm! I think I need a lens with a horizontal FoV of about 100 degrees for this shot. Now which focal length of lens will give me that? Ah! Good job I marked the horizontal FoV on this lens. 100 degrees when used on full frame 35mm. That's the lens I need."  ;D
Title: Re: Perspective Revisited
Post by: Guillermo Luijk on February 04, 2012, 11:36:55 pm
I've never met anyone who thinks a lens will change its actual  physical and material properties when attached to a different camera body, but I guess someone who believes in magic might think that.

That is good news for you because you'll learn a new thing today, people thinking a lens can change its focal length (a physical and material property of the lens) depending on the body it is attached to:

"Tengo una pequeña duda, en relación al ajuste de la distancia focal con objetivos adaptados. Por ejemplo, si utilizo un Zuiko Auto-S 50/1.8, qué pongo 50mm o 100mm ?"

I'll translate for you this question repeteadly appeared in the M4/3 forums (crop factor of 2), about which focal length to set in the camera stabilizer menu:

"I have a question, in relation to the focal length with adapted lenses. For instance, if I use a Zuiko Auto-S 50/1.8, what should I set [in the camera stabilizer menu] 50mm or 100mm ?".

If this poor guy hadn't been bombed one thousand times by people like you with things such as "M4/3 cameras provide an equivalent focal length that is twice the real focal length", he would have known that he had to set the one and only focal length of the attached lens, i.e. 50mm, never 100mm.

Regards
Title: Re: Perspective Revisited
Post by: Ray on February 05, 2012, 10:30:32 am
If this poor guy hadn't been bombed one thousand times by people like you with things such as "M4/3 cameras provide an equivalent focal length that is twice the real focal length", he would have known that he had to set the one and only focal length of the attached lens, i.e. 50mm, never 100mm.

Regards


But would he have known what minimum shutter speed to use in the absence of any image stabiliser at all, based on the frequently mentioned adage that a shutter speed of 1/FL provides reasonably sharp results, if you hold the camera steady?

Without the concept of 'focal length equivalence', that poor guy using a 4/3rds camera without built-in image stabiliser, might have discovered he'd taken a number of irreplaceable shots that were less than adequately sharp as a result of using 1/50th sec exposure with a 50mm lens instead of the 1/100th sec exposure that focal length equivalence implies.

I have no problem in the theory or of my understanding of the term 'equivalent FoV'. I just think it's more practical to use 'equivalent focal length' because all lenses are specified with a focal length reference and not an FoV reference. Not only that, I think it would be true to say that all reviews of P&S cameras, and even the manufacturers' websites, mention the 35mm format focal length equivalence of the P&S fixed lens.

Whatever system of nomenclature one adopts, there will always be a number of people who get confused by what is meant. However, I would be willing to place a bet that a change to FoV equivalence would cause greater confusion.
Title: Re: Perspective Revisited
Post by: ErikKaffehr on February 05, 2012, 11:36:06 am
Hi,

Before I was doing digital my most used camera was a Pentax 67 MF camera. I used to have four lenses, 45/4, 90/2.8, 165/2.8 and 300/4. I never thought of them  as 24, 50, 85 or 180 mm equivalents.

Best regards
Erik


But would he have known what minimum shutter speed to use in the absence of any image stabiliser at all, based on the frequently mentioned adage that a shutter speed of 1/FL provides reasonably sharp results, if you hold the camera steady?

Without the concept of 'focal length equivalence', that poor guy using a 4/3rds camera without built-in image stabiliser, might have discovered he'd taken a number of irreplaceable shots that were less than adequately sharp as a result of using 1/50th sec exposure with a 50mm lens instead of the 1/100th sec exposure that focal length equivalence implies.

I have no problem in the theory or of my understanding of the term 'equivalent FoV'. I just think it's more practical to use 'equivalent focal length' because all lenses are specified with a focal length reference and not an FoV reference. Not only that, I think it would be true to say that all reviews of P&S cameras, and even the manufacturers' websites, mention the 35mm format focal length equivalence of the P&S fixed lens.

Whatever system of nomenclature one adopts, there will always be a number of people who get confused by what is meant. However, I would be willing to place a bet that a change to FoV equivalence would cause greater confusion.
Title: Re: Perspective Revisited
Post by: Guillermo Luijk on February 05, 2012, 12:43:37 pm
Before I was doing digital my most used camera was a Pentax 67 MF camera. I used to have four lenses, 45/4, 90/2.8, 165/2.8 and 300/4. I never thought of them  as 24, 50, 85 or 180 mm equivalents.

For the same reason (just in the opposite direction), when I used a 50mm on my first DLSR, an APS-C sized Canon 350D, I never thought of it as an 80mm equivalent. It was a 50mm on an APS-C camera, and I knew its expected FOV since that sensor size became my natural reference.
Title: Re: Perspective Revisited
Post by: Ray on February 05, 2012, 02:39:31 pm
Hi,

Before I was doing digital my most used camera was a Pentax 67 MF camera. I used to have four lenses, 45/4, 90/2.8, 165/2.8 and 300/4. I never thought of them  as 24, 50, 85 or 180 mm equivalents.

Best regards
Erik



Well, I would guess you're in a minority, Erik.

Before I was doing digital, my most used cameras were all 35mm format without exception. There was little reason for me to think of them as MF equivalents, but I was nevertheless aware that bigger cameras used longer focal lengths of lens to achieve the same FoV, and that seemed a disadvantage to me, as well as the additional cost, bulk and weight.

The facts are, the 35mm camera, prior to the introduction of digital cameras, was the most popular camera format ever. The first DSLRs for a number of years were cropped versions of the 35mm format for cost reasons, but they all used 35mm format lenses, so it became necessary to completely understand the significance of focal length equivalence, for very sound practical reasons.

The issue has also been compounded by the obscure terminology used to describe the size of the sensors in P&S cameras, such 1/1.7" or 1/2.3" or 2/3" etc., which means nothing to the average buyer of these cameras.

It's always useful and helpful, and even necessary to have some reference point or standard. Having chosen a standard reference point for sound practical reasons, such as the 35mm format standard, it becomes difficult and perhaps somewhat pointless to change it.

I happen to have with me a Lumix DMC-FT1 P&S which came as a freebie when I bought my Panasonic plasma HDTV. I don't use it much. Out of curiosity I'm now looking to see if the focal length of the zoom is mentioned on the camera body. It isn't. All that's mentioned is 28mm wide, the 35mm format equivalent. However, on the specifications page of the manual, both the actual and 35mm equivalent range of the zoom is mentioned, 4.9mm to 22.8mm and 28mm to 128mm.
Title: Re: Perspective Revisited
Post by: Wayne Fox on February 05, 2012, 05:55:01 pm
Well, I would guess you're in a minority, Erik.

I don't think so.

I also shot MF film and I also never worried about equivalent focal lengths. I really don't think anyone did.

 I don't people think about it that much now, except when moving to a new format and getting a handle on what lens will deliver equivalent FoV to the lenses they are used to on their previous format.  Once they've got that figured out (unless you use a really small sensor) they just use the actual focal length to describe their lenses.
Title: Re: Perspective Revisited
Post by: Bart_van_der_Wolf on February 05, 2012, 07:13:24 pm
I don't think so.

I also shot MF film and I also never worried about equivalent focal lengths.

Hi Wayne,

Same with me, 6x7cm and 6x6cm (and 4x5 inch). One just knows (after a while) intuitively which lens gives the anticipated field of view.

Cheers,
Bart
Title: Re: Perspective Revisited
Post by: ErikKaffehr on February 05, 2012, 07:50:21 pm
Hi,

Yes, on the other hand, with the new compacts there are a lot of different sensor sizes, so it may be practical to use 35 mil film as a reference.

BR
Erik


Hi Wayne,

Same with me, 6x7cm and 6x6cm (and 4x5 inch). One just knows (after a while) intuitively which lens gives the anticipated field of view.

Cheers,
Bart
Title: Re: Perspective Revisited
Post by: Bart_van_der_Wolf on February 05, 2012, 08:15:19 pm
Yes, on the other hand, with the new compacts there are a lot of different sensor sizes, so it may be practical to use 35 mil film as a reference.

Hi Erik,

I'm not so sure anymore. My smartphone has (for picture taking, not video conferencing) a 4.31mm focal length. That does't tell me much if I don't know the field of view that it apparently covers due to the tiny 8MP sensor. Maybe Field of View is the new more logical cross-platform reference (once we can agree on the aspect ratio ... ;) ).

Cheers,
Bart
Title: Re: Perspective Revisited
Post by: Ray on February 06, 2012, 12:52:50 pm
I don't think so.

I also shot MF film and I also never worried about equivalent focal lengths. I really don't think anyone did.

 I don't people think about it that much now, except when moving to a new format and getting a handle on what lens will deliver equivalent FoV to the lenses they are used to on their previous format.  Once they've got that figured out (unless you use a really small sensor) they just use the actual focal length to describe their lenses.

Clearly if you use only one format, you are less likely to be concerned about the equivalent focal lengths for another format. It would only be of academic interest.

However, my understanding is that serious photographers who would frequently change formats when choosing the best tool for the job or for the circumstances, would be aware of the focal length equivalents for the different formats they used.

I mentioned that I used  35mm exclusively before moving to digital. That's not quite true. I made the mistake of buying a couple of second-hand MF cameras at the time professional photographers were dumping their MF film equipment in favour of the new digital cameras. I guess I was a sucker for a bargain. A Mamiya RB67 with 75mm and 360mm lenses, plus a Fuji GW690 with fixed 50mm lens, cost me less than the price I would have paid for that first Canon 3mp DSLR body only, the D30, which I recall was priced around A$6,000.

I chose the lenses with reference to the 35mm format lenses I was familiar with. Using the concept of focal length equivalence, I was aware that the fixed 50mm lens on the GW690 was a wide-angle lens, that the 75mm on the RB67 was a little bit wide, not quite the standard lens, and that the 360mm was only a very moderate telephoto in 35mm terms.

At the time, the purchase seemed a no-brainer. However, I didn't realise that within a year Canon would produce an upgrade to the D30 with double the pixel count at a lower price. The convenience of the D60 which I bought a few months later made those MF film cameras almost museum pieces from the start.

The following site provides a useful list of FL equivalents for 35mm, 4x5 and 8x10, and talks about the different size image circles which lenses of equal and actual focal lengths can have, which is a major consideration when using a lens designed for 4x5 format as a wide-angle lens for 8x10 format. 

http://www.toyoview.com/LensSelection/lensselect.html

Approximate equivalents of lens focal length

 
35mm   4x5   8x10      

20mm   65mm   120mm      
24mm   75mm   155mm      
28mm   90mm   200mm      
35mm   115mm   240mm      
45mm   150mm   300mm      
52mm   180mm   360mm      
63mm   210mm   420mm      
90mm   300mm   600mm      
105mm   360mm   720mm      
135mm   480mm   900mm   
Title: Re: Perspective Revisited
Post by: Ray on February 06, 2012, 12:58:31 pm
Same with me, 6x7cm and 6x6cm (and 4x5 inch). One just knows (after a while) intuitively which lens gives the anticipated field of view.


I am surprised, Bart. I would have thought a person like you who is so technically competent and knowledgeable would find out immediately which lens gave the anticipated field of view, rather than wait for intuition after a period of trial and error.  ;D
Title: Re: Perspective Revisited
Post by: Ray on February 06, 2012, 01:58:45 pm
Because I'm familiar with looking at photographs and the properties of lenses, the relative size of things in a telephoto image (what's called "compression") gives me a clue that the things in the photograph were actually quite distant from the camera. So the spacial perception of the photograph can vary from viewer to viewer - which is anything but clear ;)

That's an excellent point which I'll address. I believe it is true that an experienced photographer may notice the slight compression distortion in a telephoto shot and realise that the subject is not in fact as close as it may seem to the uncritical eye.

The same applies to the wide-angle shot. The experienced photographer may realise that the distant objects that appear really far away are in fact not as far as they appear. There may be clues that a wide-angle lens was used which, as we all know produces 'extension distortion'.

Now consider the following. It's a significant point. The extension distortion of the wide-angle shot, which makes distant objects look more distant than they actually are, is transformed into the compression distortion of the telephoto shot, which makes close objects look closer than they actually are, simply by cropping both images to the same FoV.

How come? Can't you see the absurd contradiction here? Those who claim that FoV or focal length of lens have no bearing on perspective, and that only position affects perspective, must now concede that there's no such thing as extension and compression distortion. It's all an illusion.

How can one change the extension distortion in a wide-angle shot to the compression distortion of a telephoto shot and simulataneously claim that perspective has not changed?

Does anyone really believe this is merely semantics?
Title: Re: Perspective Revisited
Post by: 32BT on February 06, 2012, 05:57:51 pm
Now consider the following. It's a significant point. The extension distortion of the wide-angle shot, which makes distant objects look more distant than they actually are, is transformed into the compression distortion of the telephoto shot, which makes close objects look closer than they actually are, simply by cropping both images to the same FoV.

How come? Can't you see the absurd contradiction here? Those who claim that FoV or focal length of lens have no bearing on perspective, and that only position affects perspective, must now concede that there's no such thing as extension and compression distortion. It's all an illusion.

How can one change the extension distortion in a wide-angle shot to the compression distortion of a telephoto shot and simulataneously claim that perspective has not changed?

Does anyone really believe this is merely semantics?


Don't be ridiculous Ray,

You don't crop both images, you crop the wide-angle shot to an equivalent field-of-view or angle-of-view as the telephoto shot and because the angle of incident light is then equivalent, so is the apparent distortion. if you subsequently blow up the cropped wide-angle shot to the exact scale of the telephoto shot, it will exhibit the exact same compression.

The compression and expansion are not constant over the frame. It expands because it becomes wider than what would be considered a normal lens, and it compresses when the angle is narrower for a telelens. In a telelens, as well as in the center part of the wide-angle lens, incident light rays are increasingly more parallel. Thus it will look more like a parallel projection.



Title: Re: Perspective Revisited
Post by: Guillermo Luijk on February 06, 2012, 06:48:19 pm
Does anyone really believe this is merely semantics?

Basically everyone but you.
Title: Re: Perspective Revisited
Post by: Fine_Art on February 07, 2012, 12:53:56 am
That's an excellent point which I'll address. I believe it is true that an experienced photographer may notice the slight compression distortion in a telephoto shot and realise that the subject is not in fact as close as it may seem to the uncritical eye.

The same applies to the wide-angle shot. The experienced photographer may realise that the distant objects that appear really far away are in fact not as far as they appear. There may be clues that a wide-angle lens was used which, as we all know produces 'extension distortion'.

Now consider the following. It's a significant point. The extension distortion of the wide-angle shot, which makes distant objects look more distant than they actually are, is transformed into the compression distortion of the telephoto shot, which makes close objects look closer than they actually are, simply by cropping both images to the same FoV.

How come? Can't you see the absurd contradiction here? Those who claim that FoV or focal length of lens have no bearing on perspective, and that only position affects perspective, must now concede that there's no such thing as extension and compression distortion. It's all an illusion.

How can one change the extension distortion in a wide-angle shot to the compression distortion of a telephoto shot and simulataneously claim that perspective has not changed?

Does anyone really believe this is merely semantics?

Yes.

Do you really believe light starts bending around objects to compress or extend when you change focal length? For any lens, excluding diffraction differences, the light from a point in the scene travels a straight line into the lens. The only difference is how wide an angle of light does the lens let in.
Title: Re: Perspective Revisited
Post by: Ray on February 07, 2012, 01:00:00 am

Don't be ridiculous Ray,


Don't be ridiculous, Oscar.  All images are cropped, whether in-camera or through post processing. If the telephoto image had not been cropped you'd see a circle with a very dark circumference, and that would be impossible to emulate by cropping the wide-angle shot.  ;D


Quote
.....you crop the wide-angle shot to an equivalent field-of-view or angle-of-view as the telephoto shot and because the angle of incident light is then equivalent, so is the apparent distortion. if you subsequently blow up the cropped wide-angle shot to the exact scale of the telephoto shot, it will exhibit the exact same compression.

Eh! ...the angle of incident light is then equivalent?? Surely the angle of incident light is/was always the same in respect of the same objects in the scene, whatever the lens used. It's not changed through cropping. What has changed are the reference points that provide the impression of distance. They've been removed and the ball game has consequently changed.

Quote
The compression and expansion are not constant over the frame. It expands because it becomes wider than what would be considered a normal lens, and it compresses when the angle is narrower for a telelens. In a telelens, as well as in the center part of the wide-angle lens, incident light rays are increasingly more parallel. Thus it will look more like a parallel projection.


That sounds a bit confused to me. Are you introducing the red herring of volume anamorphosis again? Let's just shift focal lengths upwards by a few mm, and compare a standard 50mm shot with a significantly telescopic shot.

The 50mm shot provides no sense of either compression distortion or extension distortion. It looks pretty close to what the eye sees in respect of the relative size and distances of objects in the scene.

We then make a crop of the central area of the 50mm shot to emulate the focal length of the telephoto lens. We don't emulate the FoV. We make it exactly the same. It's not an equivalent FoV. It's the same FoV. However, the same FoV results in a focal length of lens which is equivalent to the telephoto lens used. We've demonstrated the principle that lenses of equivalent focal length will produce an identical sense of perspective from the same position.

I presume by expansion you mean extension. The extension distortion in a wide-angle shot refers to the effect or impression of extended distances. The compression distortion of a telephoto shot refers to the effect of shortened distances.

How do we make something that previously looked small (and distant) appear large and close? By removing the big things in the scene (through cropping) and magnifying the small things.

The principle here is that big and small are relative terms. Big is big only in relation to something that is small, and vice versa. Is an elephant big? Not compared with a whale.
Title: Re: Perspective Revisited
Post by: Ray on February 07, 2012, 01:06:23 am
Basically everyone but you.


Sounds as though I might be right then, on the basis that the majority is often wrong.  ;D
Title: Re: Perspective Revisited
Post by: 32BT on February 07, 2012, 03:58:54 am

I presume by expansion you mean extension. The extension distortion in a wide-angle shot refers to the effect or impression of extended distances. The compression distortion of a telephoto shot refers to the effect of shortened distances.


As long as we agree that these are not a fixed distortion as a property of a lens. It is an apparent distortion as a result of the angle of light rays being captured. It is therefore not fixed across the frame. Perspective appears to extend for angles larger than normal, and it appears to compress for angles smaller than normal. All within the same lens.

And yes, it is somewhat similar to the previously mentioned volume anamorphosis, because in a parallel projection, objects of the same size will be drawn with the same size, even though they are at different distances from the point-of-view.



How do we make something that previously looked small (and distant) appear large and close? By removing the big things in the scene (through cropping) and magnifying the small things.

Okay, I get what you mean. But you previously mentioned making prints. If the wide-angle print is printed large enough so that the angle-of-view when viewed at a reasonable distance becomes equivalent to "normal", and we also look at a telephoto print where the objects of interest are the same size as the wide-angle print. Would we then still judge the objects as being at different distances or of different size?



Title: Re: Perspective Revisited
Post by: Ray on February 07, 2012, 01:43:45 pm
Okay, I get what you mean. But you previously mentioned making prints. If the wide-angle print is printed large enough so that the angle-of-view when viewed at a reasonable distance becomes equivalent to "normal", and we also look at a telephoto print where the objects of interest are the same size as the wide-angle print. Would we then still judge the objects as being at different distances or of different size?

Probably not. It would be interesting to do the experiment involving viewers who were not aware of these issues we've discussed. When viewing such prints side by side, from the same distance appropriate for the much larger wide-angle shot but not appropriate for the small crop, the small crop, say postcard size or A4, would probably be recognised as a crop from the centre of the wide-angle shot, and the perspective should appear the same. However, I suspect this wouldn't be the case if the smaller print were hand-held and viewed like a postcard, and the larger print viewed from a greater distance.
Title: Re: Perspective Revisited
Post by: Wayne Fox on February 07, 2012, 02:24:10 pm
Basically everyone but you.

and we're all feeling like this about now ...
Title: Re: Perspective Revisited
Post by: Ray on February 08, 2012, 04:42:30 am
and we're all feeling like this about now ...


Wayne,
Some of us like to tell it as we see it, without fear of ridicule. Others prefer the herd mentality. I wonder if the problem is that some of you guys are stuck in the Aristotelian 'either/or' concept. That is, either position determines perspective, or it doesn't. I've never argued that it doesn't. I'm arguing that there are additional factors.
Title: Re: Perspective Revisited
Post by: Guillermo Luijk on February 08, 2012, 08:35:17 am
and we're all feeling like this about now ...

Nice GIF, I have another image for these situations:

(http://www.guillermoluijk.com/misc/maemia.jpg)
Title: Re: Perspective Revisited
Post by: Eric Myrvaagnes on February 08, 2012, 10:04:37 am
Yes, Ray, we know that you prefer the Humpty-Dumpty concept to the Aristotelian one. I think Wayne and Guillermo have expressed (in images) my own feeling at this point very well.

My one remaining suggestion is this: Why don't you coin a new term for your version of "perspective" which differs so greatly from the accepted meaning. How about "Rayperspective" for your version and simple "perspective" for the rest of us.

Eric
Title: Re: Perspective Revisited
Post by: Wayne Fox on February 08, 2012, 02:54:55 pm
Wayne,
Some of us like to tell it as we see it, without fear of ridicule. Others prefer the herd mentality. I wonder if the problem is that some of you guys are stuck in the Aristotelian 'either/or' concept. That is, either position determines perspective, or it doesn't. I've never argued that it doesn't. I'm arguing that there are additional factors.


Not trying to ridicule ... the graphic is exactly how I feel.  I'm a pretty laid back person, as my history of posting on this forum shows.  And I really should move on, but I guess I'm just totally amazed by your continuing to try and support your own personal definition on a concept that is very well understood by experienced photographers.

As I've mentioned,  there is some logic to your point of view, but in fact defining the concept in the traditional manner and leaving the other concepts on their own is useful and helpful.  So you refuse to accept the "standard".  Nothing about a "herd" mentality here, it's helpful to have a standard that defines a meaning as it relates to a particular application.

When I'm teaching photography, do I teach it your way?  Or do I teach it the way everyone else understands it? Or do I have to teach it both ways, and then let the student decide their definition?  Sorry, but defining it the way it is currently accepted is easy to teach, and keeping it separate from other elements you are wanting to include with it makes them easy to teach as well.

To me the the accepted concept of perspective in photography is useful, is a standard, and has no need of redefinition.  To expect that you alone see the "correct" definition and the entire field of photography is wrong and should redefine the concept is a ridiculous.

Your persistence is puzzling.  You are not going to change anyones mind ... so go ahead and believe what you want.

So best of luck ... I'm done with this one.

Title: Re: Perspective Revisited
Post by: ErikKaffehr on February 08, 2012, 02:57:50 pm
Hi Eric,

I would say the word is "Rayspective". By the way, here in the north "Per" is a very common name, so we can have Perspective, too.

Best regards
Erik


Yes, Ray, we know that you prefer the Humpty-Dumpty concept to the Aristotelian one. I think Wayne and Guillermo have expressed (in images) my own feeling at this point very well.

My one remaining suggestion is this: Why don't you coin a new term for your version of "perspective" which differs so greatly from the accepted meaning. How about "Rayperspective" for your version and simple "perspective" for the rest of us.

Eric
Title: Re: Perspective Revisited
Post by: fredjeang on February 08, 2012, 03:04:40 pm
Hi Eric,

I would say the word is "Rayspective". By the way, here in the north "Per" is a very common name, so we can have Perspective, too.

Best regards
Erik



Erik, this one is excelent !!

"Rayspective". it should be pattented.
Title: Re: Perspective Revisited
Post by: theguywitha645d on February 08, 2012, 04:05:29 pm
My very last post on perspective.

The two primary factors that impact (linear) perspective are object distance (true perspective) and viewing distance (apparent perspective). It is the viewing distance that shows differences in focal length and angle of view (if perspective, which is defined as the depth in a 2-D image, did not change, how could we notice what images were taken with wide angles and telephotos--they would look the same). So just as where you stand is important for creating the perspective, where you stand when viewing the perspective is important as well--perspective is a type of projection system.

Apparent perspective will be the same if the image is viewed in relation to the focal length and magnification of the image--you view wide-angle image closer and telephoto image further away (the would be defined as the "Correct" viewing distance). However, we usually view images at a fixed distance. If that distance is further away then the Correct viewing distance, then the apparent perspective is stronger. If it is closer, then it is weaker. "Standard" viewing distance is proportional to the diagonal of the image and so the standard and correct viewing distance are the same in that case, that is why we call a lens with a focal length equal to the format diagonal "Normal"--it has nothing to do with angle of view being the same as the eye, but rather giving the natural perspective when viewed at the standard distance.

When the actual viewing distance is very different from the Correct viewing distance, the image will appear distorted. The Wide-Angle Effect is well known where heads are stretched away from the optical axis, but if you place your eye close to the image at the correct viewing distance, the head will appear round--same thing with writing on roads. When you have very long focal lengths, you get very unnatural compression of features.

There are tons of books on perspective--I have read a whole bunch of them. Common references include The Focal Encyclopedia of Photography, Third Edition, Materials and Processes of Photography, and View Camera Techniques. Here is an online reference:

http://books.google.com/books?id=hcq_40I_7egC&pg=PA12&lpg=PA12&dq=true+perspective+optics+in+photography&source=bl&ots=qU_Jq03dvX&sig=qQVd1v0A0yoPDMYTJQ76mTOFhQ8&hl=en&sa=X&ei=gQQoT5PeN-nw0gGa5qCqAg&ved=0CDEQ6AEwAg#v=onepage&q=true%20perspective%20optics%20in%20photography&f=false
Title: Re: Perspective Revisited
Post by: Eric Myrvaagnes on February 08, 2012, 11:46:10 pm
Hi Eric,

I would say the word is "Rayspective". By the way, here in the north "Per" is a very common name, so we can have Perspective, too.

Best regards
Erik


Absolutely, Erik!   ;D

Eric
Title: Re: Perspective Revisited
Post by: Ray on February 09, 2012, 01:27:10 am
Yes, Ray, we know that you prefer the Humpty-Dumpty concept to the Aristotelian one. I think Wayne and Guillermo have expressed (in images) my own feeling at this point very well.

My one remaining suggestion is this: Why don't you coin a new term for your version of "perspective" which differs so greatly from the accepted meaning. How about "Rayperspective" for your version and simple "perspective" for the rest of us.

Eric

Quote
Why isn't the burden on you [Ray] to make clear what you are referring to when you talk of perspective? Why is it that you [Ray] own the definition of that word?

Eric and Isaac,
I have no need to coin a new term for perspective. It's already been defined in the Oxford English Dictionary. That's the definition that I've been using all along.

It seems that in this thread, I'm the only one who has provided a definition to make it clear what what I'm referring to. It's no wonder you guys are banging your head against the keyboard. You're without a comprehensive definition.  ;D

To help alleviate your frustration, I'll provide the definition again, from the most authoritative dictionary of the English language. It even provides Americanisms.

Quote
PERSPECTIVE

“The art of drawing solid objects on a plane surface so as to give the same impression of relative position, size, or distance, as the actual objects do when viewed from a particular point.”

On the way to the airport yesterday, I asked the taxi driver to stop at a temple we were passing, so I could take a few quick shots.

As I raised the D700 to my eye, with lens set at 14mm, I got a shock. "Oh my Gawd!, I exclaimed. Some of those buildings look so far away. I haven't got the time to walk over and have a closer look. I might miss my flight."

However, when I lowered the camera, I could see with my naked eyes (although still wearing clothes, of course) that some of the temple buildings were not nearly as distant as they appeared through the 14mm lens, so I relaxed and my heart beat slowed down. I wasn't going to miss my flight.

Now the Oxford dictionary refers specifically to drawing in its definition. To include photography and the concept of focal length, we could rephrase the definition as follows, without changing the underlying principle.

Quote
The art of depicting and presenting solid objects on a plane surface so as to give the same impression of relative position. size, or distance, as the actual objects do when viewed from a particular point with the naked eye or when viewed through a standard camera lens of focal length approximately equal to the diagonal of the sensor.

In the image below I didn't use an actual 50mm lens. I used a 50mm equivalent, which provided the same impression of distance that I actually saw with my naked eyes.

Title: Re: Perspective Revisited
Post by: Eric Myrvaagnes on February 09, 2012, 09:24:51 am
What frustration? If quarrelling about definitions makes your day then go right ahead.
+10!
Title: Re: Perspective Revisited
Post by: theguywitha645d on February 09, 2012, 11:49:46 am
To help alleviate your frustration, I'll provide the definition again, from the most authoritative dictionary of the English language. It even provides Americanisms.

OK, I lied, I will make another post. Here is another definition:

From The focal Encyclopedia of Photography:

Quote
PERSPECTIVE Persepctive refers to the appearance of depth when a three-dimensional object or scene is represented in a two-dimensional image, such as a photograph, or when the subject is viewed directly.

The difference you are seeing in the examples you are posting is simply the difference in the "correct" viewing distance--the correct viewing distance is different for each image, but you are using the same viewing distance for both and so the perspective is different. Viewing distance is simply changing apparent perspective. While you are seeing a relationship between focal length and perspective, it is an indirect relationship. Viewing distance actually gives the final perspective because you can actually get the same perspective at different distances and with different focal lengths, so those are not determining how we finally view an image--viewing distance also explains why we perceive the crop different from the whole because the whole will have the same perspective as the crop if magnified in relation to the crop. See my post above.
Title: Re: Perspective Revisited
Post by: Ray on February 10, 2012, 04:25:17 am

The difference you are seeing in the examples you are posting is simply the difference in the "correct" viewing distance--the correct viewing distance is different for each image, but you are using the same viewing distance for both and so the perspective is different. Viewing distance is simply changing apparent perspective. While you are seeing a relationship between focal length and perspective, it is an indirect relationship. Viewing distance actually gives the final perspective because you can actually get the same perspective at different distances and with different focal lengths, so those are not determining how we finally view an image--viewing distance also explains why we perceive the crop different from the whole because the whole will have the same perspective as the crop if magnified in relation to the crop. See my post above.

Not at all. My approach is rational and scientific. I fully realise the importance of the 'correct' viewing distance and make sure I've got it.

I'm surprised that certain individuals who have commented in this thread, and who appear to have a fairly sound scientific background, are experiencing such difficulty with this concept of perspective.

Surely everyone with at least a basic knowledge of scientific procedure and methodology should understand that in order to determine the effect of any change in a particular parameter, one should at least try to keep everything else the same. The only change, if possible, should be in the parameter under examination, in this case focal length of lens.

If we wish to examine what a change in position has on perspective, then we should change only the shooting position, either up or down, from one side to another, or forwards or backwards, and keep everything else the same, if possible, in order not to confuse the results with other influences, whether such influences be either direct or indirect.

We should use the same camera and lens, and view the resulting photographic images at the same size and from the same distance. We can then appreciate the precise effect that changing the shooting position has on the final result.

Likewise, if we wish to examine whether a change in focal length has any effect on perspective, we should adopt the same procedure of eliminating all variables other than lens focal length, as far as possible. We should use the same camera, but use different focal lengths of lens. We should shoot from the same position and angle, make prints of the same size and view them from the same distance. That's just plain common sense. It's not rocket science.

Having done this, it's clear to me that the choice of focal length of lens affects the sense of perspective in the resulting image, whether on print or screen. How anyone could deny that, beats me, unless he is on hallucinatory drugs.

However, I would agree there is another approach to keeping everything else the same as far as possible. Instead of keeping the print size the same we could keep any object that is common to all images under comparison, the same size. We would then have to scale the print size in inverse proportion to the focal length. In other words, the largest print would be from the shortest focal length, and the smallest print from the shot using the longest focal length.

If the largest print that one decides is practicable is, for example, 24"x36" using a 12.5mm lens (35mm format), then the print size from a shot with a 25mm lens will be 12"x18", a 50mm shot 6"x9", a 100mm shot 3"x4.5", a 200mm shot 1.5"x2.25", and a 400mm shot 0.75"x 1.125" which is the size of a rather small postage stamp.

Now if this is your method of working as a photographer, to scale print size in inverse proportion to focal length, then you are absolutely right to claim that focal length has no bearing on perspective. If we were to examine, for example, the stationary insect on a leaf which fills the small postage-stamp print, using a magnifying glass, we might notice the compression distortion that is  typical of a telephoto lens. When examining the same leaf in the centre of the 12.5mm shot, assuming we are able to find it, we might get the impression that the insect exhibits the same qualities of compression distortion. However, you'd probably need a D800, or better still an IQ180 to carry out such tests, and even then the insect might be so blurred it could be impossible to be sure if the perspective were the same.

But let's not get side-tracked by such practical difficulties. In principle, if the resolution is sufficient to compare the same size objects in different size prints, from the same close-up viewing distance, which is necessary to see small objects, then perspective should be the same, having previously eliminated lens distortions.

However, I have to admit that you guys are the only photographers I've come across who scale print size in inverse proportion to the focal length of lens used, in order to conform with the sacred rule that focal length has no bearing on perspective.  ;D

Title: Re: Perspective Revisited
Post by: fredjeang on February 10, 2012, 04:34:29 am
Guys, we all have been fooled.

Ray is a master indeed and shouldn't be underestimated.

I suspect that he knows perfectly what we all were trying to explain. But the thing is that he is playing with us, has the time to because he is retiree, travel and has fun when he reads our posts.

The C.I.A or the Mossad should hire him for certain secret provocative tasks.

He may put all of us on nerves, but we are in fact manipulated.

Title: Re: Perspective Revisited
Post by: Bart_van_der_Wolf on February 10, 2012, 05:33:04 am
The difference you are seeing in the examples you are posting is simply the difference in the "correct" viewing distance--the correct viewing distance is different for each image, but you are using the same viewing distance for both and so the perspective is different. Viewing distance is simply changing apparent perspective. While you are seeing a relationship between focal length and perspective, it is an indirect relationship. Viewing distance actually gives the final perspective because you can actually get the same perspective at different distances and with different focal lengths, so those are not determining how we finally view an image--viewing distance also explains why we perceive the crop different from the whole because the whole will have the same perspective as the crop if magnified in relation to the crop. See my post above.

Exactly, couldn't agree more. I've already explained it in another thread with a reference to anamorphic projection distortion (http://www.luminous-landscape.com/forum/index.php?topic=61388.msg496106#msg496106), or anamorphosis.

I've also outlined all elements (http://www.luminous-landscape.com/forum/index.php?topic=61388.msg496400#msg496400) that play a role from a photographic perspective (pun intended).

I don't feel an urge to repeat myself ad nauseam, hence the links for those who are interested and have more time to waste while trying to re-invent the obvious.

Cheers,
Bart
Title: Re: Perspective Revisited
Post by: theguywitha645d on February 10, 2012, 11:36:16 am
Having done this, it's clear to me that the choice of focal length of lens affects the sense of perspective in the resulting image, whether on print or screen. How anyone could deny that, beats me, unless he is on hallucinatory drugs.

And you are right, but it is not to do directly with focal length, but the viewing distance--I have made the same error and did research to figure it out. If I take a photograph from the same point with multiple formats and the focal length of each lens is proportional to the format diagonal (or the angle of view is the same), then the resulting perspective is the same. So is it the focal length? The angle of view? Both? Neither?

Viewing distance answers the question simply and easily. In order for the perspective to appear normal, not only is the taking position important, but also the viewing position. This has been well recognized in perspective drawing as shown by the invention of the viewing pinhole and also by creating perspective drawing in reference to where the viewer would stand. But it is also known that perspective is very robust in the fact we can view an image from another position and the perspective does not look unnatural--the reason the viewing pinhole did not last long.

So while you are correct in stating the focal length choice will affect the apparent perspective in the image, you are incorrect to assign it to the focal length per se as the perspective will always be the same if images are viewed by the relative position equal to the taking position. The reason we see focal length choice is we view images from a standard distance, which is relative to the image size, rather than to the taking position. Once you compare images at a standard viewing distance, the change in perspective created by focal length/angle of view becomes apparent.
Title: Re: Perspective Revisited
Post by: Ray on February 11, 2012, 12:54:34 am
And you are right, but it is not to do directly with focal length, but the viewing distance--I have made the same error and did research to figure it out. If I take a photograph from the same point with multiple formats and the focal length of each lens is proportional to the format diagonal (or the angle of view is the same), then the resulting perspective is the same. So is it the focal length? The angle of view? Both? Neither?

A focal length which is equal to the diagonal of the sensor is called a 'standard' lens, whatever the format and whatever the actual focal length that is marked on the lens. It produces an image in the viewfinder of a DSLR which depicts very closely the size, and impression of relative distances of the objects viewed, as the naked eye sees them.

I've mentioned a few times it's not the focal length per se that counts regarding changes to perspective but the equivalent focal length in relation to sensor size. It's also helpful to have a standard reference point such as '35mm format equivalent'.

Quote
So while you are correct in stating the focal length choice will affect the apparent perspective in the image, you are incorrect to assign it to the focal length per se as the perspective will always be the same if images are viewed by the relative position equal to the taking position. The reason we see focal length choice is we view images from a standard distance, which is relative to the image size, rather than to the taking position. Once you compare images at a standard viewing distance, the change in perspective created by focal length/angle of view becomes apparent.

That sounds very confused to me. Can't make head nor tail of it. It seems to me the correct viewing distance from any print is the distance which enables one to take in the entire composition within a single glance, and yet see all the essential details in the composition, such as very small objects and distant buildings etc.

Such viewing distances tend to be somewhere between 1 & 2 times the diagonal of the print. If the print is large and high resolution it may be necessary to go closer if one wishes to inspect fine texture.

Can you give me some specific examples of correct viewing distances and print sizes for the following two examples. (1) I photograph an entire tree containing a motionless lizard in the centre, using a 24mm lens (35mm format). (2) From the same position I photograph just the lizard and parts of a few surrounding branches, using a 400mm lens (35mm format).

What are the correct viewing distances in relation to print size for these two examples, in your opinion?
Title: Re: Perspective Revisited
Post by: theguywitha645d on February 11, 2012, 01:36:02 pm
What are the correct viewing distances in relation to print size for these two examples, in your opinion?


Standard viewing distance is defined as equal to the diagonal of the print/display area.

Correct viewing distance is defined as proportional to the image magnification and focal length.

With an image taken with a normal lens, the standard viewing distance and correct viewing distance are the same. The correct viewing distance would be closer for images from wide angle lenses and longer for telephotos compared to the standard viewing distance.

Since you placed two images at the same size next to each other, you have fixed the viewing distance--I will not change my position when looking at one or the other. Because that distance is held constant, you can see the impact the change in focal length or crop has on perspective between the images. But the reason for that is the correct viewing distances for each image is very different. If you scaled one image in relation to the other so that the correct viewing distance would equal the distance I am viewing the images, then the perspective would be the same, but the images would not be the same size.

And to anticipate your question, as you get closer and further away from an image, the apparent perspective changes.

BTW, this is not my opinion. If you look at my first post above, you will see a few citations as well as a link. This problem was solved a long time ago and there is nothing to suggest perspective works otherwise.
Title: Re: Perspective Revisited
Post by: Ray on February 11, 2012, 10:30:37 pm

Since you placed two images at the same size next to each other, you have fixed the viewing distance--I will not change my position when looking at one or the other. Because that distance is held constant, you can see the impact the change in focal length or crop has on perspective between the images. But the reason for that is the correct viewing distances for each image is very different. If you scaled one image in relation to the other so that the correct viewing distance would equal the distance I am viewing the images, then the perspective would be the same, but the images would not be the same size.


As in my examples in reply #111?, where I wrote:

" If the largest print that one decides is practicable is, for example, 24"x36" using a 12.5mm lens (35mm format), then the print size from a shot with a 25mm lens will be 12"x18", a 50mm shot 6"x9", a 100mm shot 3"x4.5", a 200mm shot 1.5"x2.25", and a 400mm shot 0.75"x 1.125" which is the size of a rather small postage stamp.

Now if this is your method of working as a photographer, to scale print size in inverse proportion to focal length, then you are absolutely right to claim that focal length has no bearing on perspective."


The above comment was made to highlight the absurdity of claiming that focal length, or FoV if you prefer, has no bearing on perspective.

Since it is not normal practice for photographers to scale prints in inverse proportion to the 'effective' focal length of lens used in order to maintain correct perspective; and since it is not normal practice for viewers to increase viewing distance in proportion to the 'effective' focal length of lens used in circumstances where the photographer has deliberately made a largish print of a telephoto shot, as most of them do; and since it often may not be possible to get sufficiently far away from, say, an A2 size print of a telephoto shot because the room is not large enough; and since most people when viewing a photograph wish to see the detail in the photograph, which is the whole purpose of using a telephoto lens in the first instance, I think it is reasonable to conclude that, in practice, changes in 'effective' focal length of lens used does have an impact on the perspective of the resulting image as experienced by a viewer acting sensibly and normally.

Quote
BTW, this is not my opinion. If you look at my first post above, you will see a few citations as well as a link. This problem was solved a long time ago and there is nothing to suggest perspective works otherwise.

Indeed! The science of linear perspective was addressed long before the camera and the modern lens was invented. But it's interesting that Leonardo da Vinci anticipated the conflict between the principles of linear perspective and the effect of wide-angle lenses.

I've never argued that changing position does not alter perspective. I'm arguing that the change in the Field of View which results from a change in the 'effective' focal length of lens used, also has a bearing on perspective, in practice.

Quote
And to anticipate your question, as you get closer and further away from an image, the apparent perspective changes.

But not necessarily with regard to photographing that image from different distances with different lenses. I can photograph the picture from any distance, using the appropriate lens to capture the whole picture and maintain the same perspective of objects depicted within the image, provided I am viewing the picture 'head on' and not at an oblique angle. The change in perspective experienced by the viewer, as he moves closer to, or further from the picture, is a change in perspective of the picture as an object in relation to its surroundings. The perspective of the objects depicted within the picture is fixed, whatever the viewing distance, and has been determined by the shooting position of the photographer and his choice of lens.
Title: Re: Perspective Revisited
Post by: theguywitha645d on February 12, 2012, 12:11:05 am
Ray, the two factors that impact (linear) perspective are the object distance (true perspective) when the photograph is taken and viewing distance (apparent perspective). It is the viewing distance that causes a viewer to see the change in perspective. Apparent perspective can only be seen when viewed. That has nothing to do with the focal length directly--it is a difference between the correct viewing distance which is related to the taking condition and the actual viewing distance. That is the whole story.

If you want to say the choice in focal length changes the apparent perspective in the image, that is fine. It has been common to say that wide-angles give wrong perspective and telephotos compress space. It is a nice rule of thumb. But focal length is not determining perspective--you cannot say 150mm focal length has weak perspective as you would also have to know the format size, on 8x10 it would be wide angle. So are you going to claim angle of view? Again it would also be true only if you know the viewing condition before hand.

So the underlying experience of apparent perspective is really the ratio between the correct viewing distance and the actual viewing distance. Because it is more usual to view something close to the standard viewing distance, it is easy to predict how focal length/angle of view will impact an image generally. But it does not explain that the thumbnail image that seemed to have strong perspective looks rather weak as a 5 foot print on my living room wall.

Anyway, that is my two cents. I really don't have anything to add.
Title: Re: Perspective Revisited
Post by: Eric Myrvaagnes on February 12, 2012, 08:36:12 am
I really don't have anything to add.
Nor do I.
Nor does Ray.    :D
Title: Re: Perspective Revisited
Post by: Rob C on February 12, 2012, 01:16:18 pm
Nor do I.
Nor does Ray.    :D


Hello Eric,

It was snowing here in Mallorca when I drew open the curtains this morning; it became a little more pleasant after lunch, so I took myself off to have a coffee and then I decided to invest in a walk. That was a mistake: a threesome asked me to take their picture on a cellphone (belonging, I assumed, to one of them) and that instantly (and obviously) told me that they'd been admiring my own cellpix, and so I said of course, step this way, let's get a shot against that black curtain over the mountains surrounding the Bay. The mistake was in the time that took: before I could make it back to the car I was hit by all the hail that had been lurking within that curtain. Next time, I'll copy Mr Clooney and tell them they are mistaken, that in fact, I'm only me. Oh to be normal!

Rob C
Title: Re: Perspective Revisited
Post by: Eric Myrvaagnes on February 12, 2012, 09:36:27 pm

Hello Eric,

It was snowing here in Mallorca when I drew open the curtains this morning; it became a little more pleasant after lunch, so I took myself off to have a coffee and then I decided to invest in a walk. That was a mistake: a threesome asked me to take their picture on a cellphone (belonging, I assumed, to one of them) and that instantly (and obviously) told me that they'd been admiring my own cellpix, and so I said of course, step this way, let's get a shot against that black curtain over the mountains surrounding the Bay. The mistake was in the time that took: before I could make it back to the car I was hit by all the hail that had been lurking within that curtain. Next time, I'll copy Mr Clooney and tell them they are mistaken, that in fact, I'm only me. Oh to be normal!

Rob C
Ah, Rob! The pain of Fame! You have my sympathy. I'm glad I'm (mostly) still anonymous.

Eric
Title: Re: Perspective Revisited
Post by: Rob C on February 13, 2012, 04:18:04 am
Ah, Rob! The pain of Fame! You have my sympathy. I'm glad I'm (mostly) still anonymous.

Eric


You're kidding - right? Fame has claimed us for its own: have a website and the world gives you fame or infamy, usually the latter, as in infamy, infamy, everone has it in for me!

;-)

Rob C
Title: Re: Perspective Revisited
Post by: Ray on February 13, 2012, 07:13:48 am
In summary, I'll make the following comments to see if we can reach an agreement.

Statements such as, "Focal length of lens used, or Field of View chosen for a composition at a specified shooting position, have no bearing on perspective, period", are simply wrong, or at best misleading.

A more correct statement would be, "Choice of focal length of lens (or FoV) at a fixed shooting position, does not affect the perspective in the resulting 2-dimensional image or print provided that precise viewing-distance instructions accompany such resulting prints, in accordance with the principle that viewing distance should vary in proportion to focal length of lens used and in proportion to print size, (in order to correct for the different perspective created through the use of a particular lens in conjunction with a decision to make a particular size of print), and provided that the viewers of the resulting prints comply with such instructions however inconventient and awkward such compliance may be.

Does that statement appear reasonably accurate, or do you think it should be amended?  ;D

I thought of adding: "The author of the print reserves the right to sue the viewer for non-compliance with correct viewing-distance instructions, because such non-compliance may distort the perspective in the print and reflect unfavourably on the artistic intent of the photographer."

But I guess that would be a bit unreasonable.  ;D
Title: Re: Perspective Revisited
Post by: JohnTodd on February 13, 2012, 06:01:04 pm
How about a pair of statements:

1) Perspective is solely dependent upon the relative positions of observer and subject. (i.e. the current dictionary definition.)

2) The *perception* of perspective is *additionally affected by* field of view and print viewing distance.
Title: Re: Perspective Revisited
Post by: Eric Myrvaagnes on February 13, 2012, 07:53:30 pm
How about a pair of statements:

1) Perspective is solely dependent upon the relative positions of observer and subject. (i.e. the current dictionary definition.)

2) The *perception* of perspective is *additionally affected by* field of view and print viewing distance.
That does it.
All along Ray has seemed to be trying to equate "perspective" with "perception of perspective."

Eric
Title: Re: Perspective Revisited
Post by: Ray on February 13, 2012, 09:16:30 pm
How about a pair of statements:

1) Perspective is solely dependent upon the relative positions of observer and subject. (i.e. the current dictionary definition.)

2) The *perception* of perspective is *additionally affected by* field of view and print viewing distance.

The problem I see here is that all types of perspective, whether considered natural, correct, incorrect or distorted, are perceptions. Without perception there is no perspective. The fact that linear perspective obeys the laws of geometry, does not make it less of a perception.

Distant objects appear small. They are perceived as being small. No-one believes they actually are small, unless one is looking at a world of totally unfamiliar objects. Perspective is an illusion, but an illusion which can be described and measured to some degree mathematically.
Title: Re: Perspective Revisited
Post by: JohnTodd on February 14, 2012, 10:47:04 am
Ray,

After I posted, I anticipated that you might have more to say about my possibly artificial distinction.

I realised that there are two distinct phases to consider. Geometrical perspective, part 1 of my distinction, is fixed on the camera sensor at the moment of image-making. Short of deliberately distorting the flat image post-shutter-release, the principle of geometrical perspective can be demonstrated with a ruler and does not change.

That flat image can now be cropped in various ways, printed at various sizes and those prints viewed at various distances by various observers (each of whom can bring any number of cultural assumptions to the experience.) In each case, their individual perception of the image may be different, but the *recorded image doesn't change*. The printed pixels do not shuffle about on the paper based on the distance of the viewer from the paper.
Title: Re: Perspective Revisited
Post by: Isaac on February 14, 2012, 12:15:35 pm
Distant objects appear small. They are perceived as being small. No-one believes they actually are small, unless one is looking at a world of totally unfamiliar objects.
I see distant objects that appear small. I perceive them to be small. I believe they actually are small and I'm looking at a world of totally familiar objects. Leaves 3"x1" - 40 yards distant.
Title: Re: Perspective Revisited
Post by: Ray on February 15, 2012, 03:16:59 am
Ray,
That flat image can now be cropped in various ways, printed at various sizes and those prints viewed at various distances by various observers (each of whom can bring any number of cultural assumptions to the experience.) In each case, their individual perception of the image may be different, but the *recorded image doesn't change*. The printed pixels do not shuffle about on the paper based on the distance of the viewer from the paper.


John,
If you crop the printed image, a good proportion of the printed pixels do something even more dramatic than shuffling about. They do a complete disappearance act. You think that has no effect of perspective?  ;D

There seem to be some very strange and logically absurd arguments presented in this thread in defense of the statement that a change in focal length of lens, or a change in FoV, has no bearing on perspective.

I get a sense that there is a disembodiment of the viewer taking place in such arguments. In other words, if we remove the viewer from the scene, the perspective doesn't change with changing FL of lens or changing FoV (whichever terminology you prefer.)

The classic proof of the argument that a change in focal length of lens does not affect perspective is to demonstrate that images taken with effectively equal focal lengths of lens are the same. Well, of course they are. No argument there.

To overcome this obvious non-proof, I get a sense of a lot of wriggling and squirming going on.

Guillermo claims that 'equivalent focal length' is a fiction. It doesn't exist. All we have are different Fields of View. However, that argument doesn't change the reality. I'm quite willing to use different terminology. If the terminology 'equivalent focal length' is not acceptable and you prefer FoV, then that's fine by me. I'll simply rephrase my argument that a change of FoV results in a change in perspective, knowing quite well that any change in FoV when shooting from a specific position must result from a change in FL of lens.

The very technically knowledgeable Bart, argues that it can all be explained by anamorphism, but he refuses to elaborate. He simply  implies that anamorphic distortion may be an intrinsic property of all lenses, not just the obvious volume anamorphic distortions that one sees at the edges of  very wide-angle shots, which can be corrected by converters from DXO Labs.

If this is the case, that the anamorphic distortion of all lenses, perhaps other than the 'standard' lens, contribute to a distortion of perspective, and that nothing can be done about this, then surely one has to admit that a change in focal length inevitabley results in a change in perspective, because nothing can be done to correct the anamorphism.

However, those who claim that such 'apparent' changes in perspective resulting from a change in FL or FoV, can be corrected by changing the viewing distance to a resulting print, really take the cake.  ;D

The very fact that one has to change viewing distance to the print in order to compensate for perspective changes resulting in changes in FL or FoV, is proof that a change in FL affects perspective.

Crikey! Get real you guys for Christ's sake!  ;D
Title: Re: Perspective Revisited
Post by: Guillermo Luijk on February 15, 2012, 05:15:50 am
7 pages of semantic discussion about perspective. That's an achievement by itself Ray, congratulations!  ;D
Title: Re: Perspective Revisited
Post by: Bart_van_der_Wolf on February 15, 2012, 07:17:24 am
The very technically knowledgeable Bart, argues that it can all be explained by anamorphism, but he refuses to elaborate.

I do not refuse to elaborate on something that has been known since the early Renaisance (15th century) (http://www.mos.org/sln/Leonardo/ExploringLinearPerspective.html) and is still as valid today, I just see no point in repeating the available explanations for the umpteenth time because you choose to ignore them.

Quote
He simply implies that anamorphic distortion may be an intrinsic property of all lenses, not just the obvious volume anamorphic distortions that one sees at the edges of  very wide-angle shots, which can be corrected by converters from DXO Labs.

Here you go again, misrepresenting what others have said. Amamorphic distortion (http://en.wikipedia.org/wiki/Anamorphosis) is NOT a lens property, it's a projection effect. It is very simple to understand for most, if they are willing. Even keystoning will disappear from an image, provided the output is viewed from the correct viewpoint, just like road surface marking (http://image.shutterstock.com/display_pic_with_logo/118180/118180,1227545221,2/stock-photo-bus-and-bicycle-road-signs-and-other-road-surface-marking-elements-21039304.jpg) or street paintings (http://www.julianbeever.net/pave.htm).

Because viewing from the correct position (distance and angle) is not alway practical, e.g. a wide angle shot would need to be viewed at a relatively short uncomfortable distance, or because people do not take the effort, we also have a choice to change the output projection somewhat with software (relatively easy with pano stitching software), in an attempt to compensate for using the wrong viewpoint. In fact we can even exploit the assumption that the wrong viewpoint is going to be used, by thus suggesting a different perspective than was actually fixed at the moment of capture. Hence the perceived distortion in wide-angle shots and tele shots, which only occurs due to the wrong viewing position/distance. It is perceived distortion, which you misinterpret as perspective, which it isn't.

Cheers,
Bart
Title: Re: Perspective Revisited
Post by: Ray on February 15, 2012, 12:22:42 pm
Here you go again, misrepresenting what others have said. Amamorphic distortion (http://en.wikipedia.org/wiki/Anamorphosis) is NOT a lens property, it's a projection effect.

Bart,
In case you didn't notice, this thread is about the change in perspective that results from a change in focal length of lens, yet you deny that this change takes place on the basis of some phenomenon called anamorphic distortion that is not a property of lenses.

Sounds to me you are engaging in obfuscation.

Here are some definitions of anamorphosis that relate to images:

(1) A distorted projection or perspective; especially an image distorted in such a way that it becomes visible only when viewed in a special manner.

(2) A distorted projection or drawing that looks normal from a particular angle or with a certain mirror.

The essential characteristic of anamorphism is a change in shape, not a change in size or apparent distance which a change in focal length of lens can produce.

As you probably know, there are types of lenses called anamorphic, which are designed to deliberately distort the shape of objects in order to fit a horizontally wide angle of view onto a 4:3 format. On projection, the reverse type of anamorphic lens is used to horizontally expand the unnaturally narrow objects as recorded, so that they look natural again in a widescreen format. A common type of anamorphic distortion is a 4:3 format TV transmission which is displayed on a widescreen TV without adjustment, causing all figures to appear unnaturally fat.

What this has to do with a change of FoV containing objects which are not misshapen, beats me. I really do think you need to eloborate on this.

Cheers!  Ray

Title: Re: Perspective Revisited
Post by: Bart_van_der_Wolf on February 15, 2012, 02:26:39 pm
Sounds to me you are engaging in obfuscation.

LOL, you remind me of that guy listening to the traffic alert on his car radio, as he says to himself:"What are they talking about, one ghost rider?, there are hordes of them coming my way and what's worse, they have the audacity to greet me with their headlights".

Quote
What this has to do with a change of FoV containing objects which are not misshapen, beats me. I really do think you need to eloborate on this.

Again (http://www.luminous-landscape.com/forum/index.php?topic=61388.msg496400#msg496400)?

I'll recap:
Focal length --> magnification,
sensor dimensions --> field of view,
lens entrance pupil --> perspective,
output viewing distance --> perspective distortion.

That's all there is to it. I'm sorry if the facts confuse you.

Cheers,
Bart
Title: Re: Perspective Revisited
Post by: theguywitha645d on February 15, 2012, 04:08:10 pm
In each case, their individual perception of the image may be different, but the *recorded image doesn't change*. The printed pixels do not shuffle about on the paper based on the distance of the viewer from the paper.

Are you sure about that? But maybe the pixels do not need to move. Perhaps you can help. Here is an image with two objects, A and B. The image is viewed at different distances, 1 and 2. Would the ratio of angular sizes of A and B be the same at each position? Does viewing distance change the image?
Title: Re: Perspective Revisited
Post by: theguywitha645d on February 15, 2012, 04:15:40 pm
I'll recap:
Focal length --> magnification,
sensor dimensions --> field of view,
lens entrance pupil --> perspective,
output viewing distance --> perspective distortion.

That's all there is to it. I'm sorry if the facts confuse you.

Cheers,
Bart

Focal length --> Magnification
Sensor size and focal length --> Field of view
The ratio of object distances --> ratio of image sizes --> True perspective
The difference between the "correct" viewing distance and actual viewing distance --> Apparent perspective

I think that would be more factual. It would certainly be more in line with the texts.
Title: Re: Perspective Revisited
Post by: Bart_van_der_Wolf on February 15, 2012, 06:53:01 pm
Focal length --> Magnification
Sensor size and focal length --> Field of view

That's correct, although one could say that the focal length was already given in the first line. All that the sensor does is capture a crop of the available image circle from that lens, which results in a FOV.

Quote
The ratio of object distances --> ratio of image sizes --> True perspective

It's the same thing, using a few more words to explain. True or actual Perspective is fixed for a given viewpoint (entrance pupil), and all relative sizes result from simple geometry.

Quote
The difference between the "correct" viewing distance and actual viewing distance --> Apparent perspective

Yup, no problem with that, although distance should also encompass viewing angle of the projection plane.

Quote
I think that would be more factual. It would certainly be more in line with the texts.

The issue is that there are so many texts, and they do not all have rectilinear lens photography with planar projections in mind. They often also do not account for things like keystoning, which can be 'corrected' by using the correct viewing position (at an angle versus the planar projection). All deviations from the correct viewing position results in apparent distortions or apparent perspective.

We essentially agree on the same priciples as they were already taught in the early 1400s, I was just trying to be a bit terse.

Cheers,
Bart
Title: Re: Perspective Revisited
Post by: Ray on February 16, 2012, 01:03:01 am
I'll recap:
Focal length --> magnification,
sensor dimensions --> field of view,
lens entrance pupil --> perspective,
output viewing distance --> perspective distortion.

That's all there is to it. I'm sorry if the facts confuse you.

Cheers,
Bart

Bart, I don't feel at all confused. What I see are confusing statements from people defending the position that focal length of lens or FoV have nothing to do with perspective.

Consider your above statements. They're confused. Admit it. There's certainly no doubt that sensor size can sometimes determine field of view, but sometimes it can't. It depends upon whether one is increasing sensor size or reducing it. As you've admitted in a later post, the diameter of the lens image circle limits the maximum FoV.

It's clear to me that the role of the focal length of a lens is equally important in determining field of view, if not more significant in practice than is sensor size.

Given a specific focal length of lens designed for a specific size of sensor, one can reduce FoV through cropping but rarely increase it. However, given a fixed size of sensor, one can either reduce or increase FoV in accordance with the available focal lengths of lenses that fit the camera.

To avoid this confusion and false dichotomy, by ascribing magnification to the role of the lens, and FoV to the role of the sensor, I think it's better to use the 'focal length equivalent' terminology with reference to a standard format such as 35mm format.

I believe also that thousands of people in the photographic industry would agree with me. As I mentioned before, my Panasonic P&S has a marking under the lens, '28mm wide'. This describes the FoV very well in my opinion, and in the opinion of countless others it seems.

Even for many people buying a camera for the first time, who may never have used 35mm format, the equivalent FL in 35mm format terms is still a useful reference, as long as it is understood that 24mm is wider than 28mm and that 40mm is narrower than 28mm.

Below is another confusing statement from you, in reply #138, specifically "relatives sizes result from simple geometry."

Quote
It's the same thing, using a few more words to explain. True or actual Perspective is fixed for a given viewpoint (entrance pupil), and all relative sizes result from simple geometry.

Relative sizes do not result from Geometry, whether simpler or not. Geometry is a passive system of mathematical description. It doesn't cause anything. Long before the principles of Geometry were formulated, our ancestors were aware that the smaller appearance of the size of familiar objects denoted distance. They would have had to have understood this in order to survive. Geometry merely provides an explanation for this phenomenon.

There is also another aspect of additional confusion that results from attempting to join two experiences of perspective which are clearly separate, viewing the actual and real scene through a camera lens, and viewing a resulting 2-dimensional print of that scene in a completely different environment.

These are two separate activities. Whatever I do when processing an image to make a print, and whatever decisions I make as to paper type, print size, contrast and color saturation etc, etc, etc, and whatever placing of that print on whatever wall and whatever viewing distance is chosen by the viewer, are all factors that are only tenuously connected with the original experience of perspective in the viewer and the photographer as he took the shot.

Once a print is produced, it becomes a separate 2-dimensional object in a different environment. The same principles of perspective will apply to that object as to any other 2-dimenional object in the same environment, such as a closed window or a stain on the wall.

To argue that by changing viewing distance to the print, in order to correct for an 'unnatural' or 'distorted' perspective in the original image that may have resulted from the choice of a particular focal length of lens, proves that focal length of lens has no bearing on perspective, is ridiculous.

That's pure sophistry, Bart, in the modern sense of the term.

The very fact that one could jump through such hoops when viewing a print, in order to correct for unnatural perspective, rather than just viewing the print from a usual and normal distance equal to, say 1.5x the print diagonal, tends to prove that a change in focal length or FoV without changing position, does indeed have an effect on perspective. If there's no change, there's nothing to correct.