This entire discussion is interesting for me because I see in it an example of how people can sometimes overlook the most basic requirement of scientific procedure when promulgating a particular idea which they are convinced is right.
When examining the effect of one specific change in any system, such as a change in the focal length of lens used to take a photograph, one should try to avoid simultaneously making other changes within the system, otherwise the consequences of the one change which is being examined
may be confused by the effects of the other changes.
This is clearly what has happened in these discussion about perspective and focal length of lens.
If one wishes to be rational, logical and scientific about an issue, which is what I've attempted to be in these discussions, then, when comparing two images taken with different focal lengths of lens in order to determine if there is any difference in perspective apparent in the resulting images, one should make only the one change
that one is examining.
That change in this case should be the change in FL of lens, because that's what we are examining.
The position of the photographer should remain the same. The camera body should remain the same. The lighting conditions should remain the same. The f/stop and shutter speed should remain the same. The processing, as far as reasonable, should remain the same. The final print size should remain the same, and the viewing distance to the final image or print should remain the same.
Having met these conditions, we are then in a position to see clearly what changes in perspective may have resulted from the use of different focal lengths of lenses. Or to be more precise, what changes in 'apparent'
perspective may have taken place, because it should be understood that everything about a photograph is apparent
. I'm reminded here of that anecdote about Pablo Picasso. When he was confronted by some bloke who criticised his paintings of women, claiming that they were distorted and unrealistic, unlike a photograph, Picasso asked the bloke if he had a photograph of his wife to demonstrate what he meant. The bloke pulled out a photo of his wife from his wallet. Picasso studied the photo for a while, then asked, "Surely your wife is not this small?"
Because this issue is not serious and not likely to affect anyone's health, I find it very amusing that someone would crop the wider angle shot to the same FoV as the narrower angle shot when making the comparison, or attempt to produce different size prints and/or view them from different distances in order to compensate or correct for any apparent
changes in perspective, then claim that there is really no change in perspective.
Do some people really not understand that this is tantamount to scientific fraud, destroying and/or manipulating evidence?
If one wishes to demonstrate that an image from a wider focal length of lens when cropped to the same FoV as an image from a narrower focal length, will result in the same perspective, when the shots are taken from the same position, then one clearly must crop the wider image because that's the purpose of the experiment.
The conclusions that can be drawn from such an experiment is that focal length of lens in itself
does not necessarily
have any bearing on perspective. What is critical is that the 'equivalent
' focal lengths be the same in order for perspective to be the same, and what is also demonstrated is that a wider-angle shot can always be cropped to produce the same equivalent
focal length of a narrower-angle shot. If the equivalent focal length is the same, and if the position from which the shots are taken is the same, then the perspective will be the same, excluding such issues as differences in lens distortions.
However, when photographers select a wider lens for a particular shot, it is usually not
for the purpose of cropping the image to emulate the effect of a narrower lens, with consequent loss of resolution. It is usually for the purpose of including additional elements
in the composition. At least, that's why I
select a wide lens for any particular shot.
The inclusion of other elements in the image, such as large features in the foreground, by necessity
diminishes the size of the more distant objects within the photographic composition, and the perspective of those more distant objects, in relation to the additional elements that the wider-angle lens has included in the composition, has unavoidably changed
Thus endeth the lesson in clear thinking.