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Author Topic: Perspective Revisited  (Read 34779 times)

Bart_van_der_Wolf

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Re: Perspective Revisited
« Reply #80 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
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Ray

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Re: Perspective Revisited
« Reply #81 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   
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Ray

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Re: Perspective Revisited
« Reply #82 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
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Ray

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Re: Perspective Revisited
« Reply #83 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?
« Last Edit: February 06, 2012, 02:05:32 pm by Ray »
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32BT

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Re: Perspective Revisited
« Reply #84 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.



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Guillermo Luijk

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Re: Perspective Revisited
« Reply #85 on: February 06, 2012, 06:48:19 pm »

Does anyone really believe this is merely semantics?

Basically everyone but you.

Fine_Art

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Re: Perspective Revisited
« Reply #86 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.
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Ray

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Re: Perspective Revisited
« Reply #87 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.
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Ray

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Re: Perspective Revisited
« Reply #88 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
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32BT

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Re: Perspective Revisited
« Reply #89 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?



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Ray

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Re: Perspective Revisited
« Reply #90 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.
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Wayne Fox

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Re: Perspective Revisited
« Reply #91 on: February 07, 2012, 02:24:10 pm »

Basically everyone but you.

and we're all feeling like this about now ...
« Last Edit: February 07, 2012, 02:25:59 pm by Wayne Fox »
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Ray

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Re: Perspective Revisited
« Reply #92 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.
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Guillermo Luijk

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Re: Perspective Revisited
« Reply #93 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:

Eric Myrvaagnes

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Re: Perspective Revisited
« Reply #94 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
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Wayne Fox

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Re: Perspective Revisited
« Reply #95 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.

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ErikKaffehr

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Re: Perspective Revisited
« Reply #96 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
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fredjeang

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Re: Perspective Revisited
« Reply #97 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.
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theguywitha645d

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Re: Perspective Revisited
« Reply #98 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
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Eric Myrvaagnes

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Re: Perspective Revisited
« Reply #99 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
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