focal length ≠ perspective (optics 101)

Started Mar 19, 2009 | Discussions thread
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arachnophilia Veteran Member • Posts: 3,352
focal length ≠ perspective (optics 101)

from here: http://forums.dpreview.com/forums/read.asp?forum=1030&message=31330339

i contend that perspective is simply a matter of distance, and focal length is irrelevant. to prove such a point, i invite anyone to do the following demonstration.

set up two targets such that both can appear in the same frame, and you can gauge the relative size of each. separate the two by several feet, in the direction of the camera.

find a telephoto lens and a wide angle lens.

make two exposures, taken at the same distance, one with each lens. crop the two so that the foreground target is the same size in each frame, for easy comparison. the background target should be the same size, relative to the foreground.

if you'd like, make another two at a new distance, using the same technique. bonus points if you can get new tele to have the same foreground composition as the wide. the two wide shots should now have completely dissimilar perspective, as should the two teles, but the two shot at the same distance will still look the same.

this is why "crop factor" or "lens equivalence" works. to maintain the same angle of view on a smaller frame, you need a shorter focal length -- a 28mm lens on DX (normal) will yield the same perspective as a 43mm lens on FX (also normal), or a much much shorter focal length on a P+S. it's all equivalent, relative to the size. the only thing that makes a wide lens "wide" is the angle of view -- perspective has nothing to do with any of the physical properties of the lens.

perspective, afterall, is the apparent compression of space relative to distance. so this whole thing is kind of tautological and "duh" if you know what you're talking about. our eyes see with perspective (though not the rectilinear projection photographs entail -- which is not perspective, but rather projective distortion). looking down a road, the lamp posts near the end of the street should seem closer together than the lamp posts on your end. zooming in, and magnifying that section of the street does nothing particularly special -- the apparent spatial compression happens because of the distance. and that part of the street looks exactly the same in the wider view of the same scene. if you walk in closer to get a better view, you get a different perspective.

but photonut2008 contends differently. and he posted pictures to "prove" it. of course, using perspective control lenses to, you know, correct the perspective, is sort of cheating. wouldn't you say? would anyone ELSE like to explain to him why he's wrong? with pictures, if you'd like. i've shown him some (pretty bad) test samples, but maybe i'll go take some prettier real-world examples.

i realize this can be a little counter-intuitive, when we're taught things about flattering perspective and longer lenses for portraits, but that's just a product of the working distance, thanks to the tight framing. we're photographers; we should know about these things and get our optical facts straight. right?

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