Perspective Revisited

[ErikKaffehr]

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.

[Ray]

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

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? 

[Ray]

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  . I also realise that a camera is a camera and not a pencil-sharpener, for example.

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.

[ErikKaffehr]

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

[Guillermo Luijk]

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.

[Bart_van_der_Wolf]

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

[ErikKaffehr]

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

[Ray]

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  .

[Fine_Art]

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]


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.

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

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.

[j-land]

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

[Wayne Fox]

"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 ...

[Ray]

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." 

[Guillermo Luijk]

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

[Ray]

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.

[ErikKaffehr]

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.

[Guillermo Luijk]

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.

[Ray]

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.

[Wayne Fox]

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.

[Bart_van_der_Wolf]

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

[ErikKaffehr]

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