Originally posted by timo Actually I have never really understood what perspective distortion is supposed to be
The concept is quite simple. Images taken with a telephoto lens can create an optical illusion in which objects depicted can appear closer together than they actually were. And the opposite for wide angle lenses.
Quote: Field of view = 'crop' varies according to focal length and/or sensor size, as does DOF, but the perspective doesn't.
Precisely why the illusion exists. Someone looking at the image doesn't know right away it was taken with a telephoto lens. So when he seems objects depicted a certain size in the frame, he mistakenly believes the photographer was closer to the objects than he actually was - in the absence of information to the contrary, he will try to process the image as if it were taken from whatever distance would have been necessary with a normal lens to render the subject that size.. But if the photographer had been that close to the subject, then the perspective would have different. The viewer sees an image taken from one position - with the perspective that implies - but sees it as if it were taken from a diffeent position, which should have have a different perspective. This mismatch in perspectives is what creates the illusion.
Quote: The foreshortening effect of telephoto lenses is no different from what you would see if you cut a small rectangular hole in a piece of card and looked through it at a distant scene.
No, not true at all. Again, the illusion absolutely dpends on an actual image being printed or displayed somehow, and the angle subtended by the print/display of the image not matching the AOV depicted in the image. You can't make that happen without actually printing , displaying, or projecting an image. There is no way to reproduce that effect just by looking at the image live.
Similarly if you take a landscape shot with a wide-angle lens, and crop a small bit out of the middle, and resize it up, this mimics the effect of a telephoto lens (forget resolution for a minute) and you will see a similar 'foreshortening' effect. But there's no difference in perspective. Nothing in the image has 'moved' with respect to anything else in the image.
Quote: I understand the sort of distortion that occurs when a wide-angle view has to be mapped in rectilinear form - i.e. stretched out corners - but that's something different.
Yes, and that is what the link Anvh gave earlier deals with.
Quote: All this stuff about 'standard' lenses is a lot of hooey - the old 50mm standard is a convention which happens to be quite convenient from a compositional point of view on 35mm film
Yes, for one very important reason - it creates an AOV that is similar to the angle that is subtended to by a ttpical print viewed from a typical distance. With the result that the image produced is devoid of perspctive distortion, and is perceived as looking "natural" - as if the framed image on the wall were just a window looking out on the scene itself.
Quote: to claim that 50mm on an APS-C sensor has analogous qualities (as some have done on occasion) makes no sense at all. And the size and/or magnification of the viewfinder is irrelevant.
That much is all true, at least as regards persepctive distortion. It is a 35mm lens on APS-Cthat creates the same AOV as a 50mm lens on 135 format, and hence possesses this quality. And size / magnification of the viewfinder plays no role in this. But viewfinder size / magnification does play a role in the *other* attribute that is often erroneously attributed to normal lenses: the "fact" that the apparent size of objects in the viewfinder is the same as that when viewed with the naked eye. This only happens to be true if the viewfinder happens to be of the right magnification to achieve that effect. This does often happen to be more or less true with viewfinders for 135 format cameras, but it is hardly ever true for viewfinders on other formats. Which does make the 135 kind of special.