adamroberts wrote:The compression of the foreground to background is a property of the focal length.
For example: If you put a 50mm on a 5D and a 35mm on a 7D the FOV will be similar but the "look" will not be the same as the compression of the foreground to background will be greater on the 5D.
Sorry, but this is incorrect. The compression of the foreground to background (known as perspective distortion or, more precisely, axial magnification) is a property of the position of the observer (in our case, a camera) in space, and which direction that observer (camera) is pointing.
The "look" will be exactly the same assuming the FOV is the same and you are shooting from the same spot.
This isn't even necessarily related to lenses, but to image projection and 3-point perspective in general (of which lenses are one method).
If you don't believe me and have a 3D rendering program available that can simulate physical cameras, you can setup a scene and try it. I did, and as you can see, cropping an image is identical to using a different focal length lens
from the same point in space:
(click to make big - sensor size of this "camera" is a full frame, ~36mm horizontal)
Wikipedia explains it best (my emphasis):
Note that perspective distortion is caused by distance, not by the lens per se – two shots of the same scene from the same distance will exhibit identical perspective distortion, regardless of lens used. However, since wide-angle lenses have a wider field of view, they are generally used from closer, while telephoto lenses have a narrower field of view and are generally used from farther away. For example, if standing at a distance so that a normal lens captures someone's face, a shot with a wide-angle lens or telephoto lens from the same distance will have exactly the same perspective on the face, though the wide-angle lens may fit the entire body into the shot, while the telephoto lens captures only the nose. However, crops of these three images with the same coverage will yield the same perspective distortion – the nose will look the same in all three. Conversely, if all three lenses are used from distances such that the face fills the field, the wide-angle will be used from closer, making the nose larger, and the telephoto will be used from farther, making the nose smaller.
There may be minor differences between a crop and an equivalent focal length in the real-world due to vignetting, lens distortions, etc., but they're going to be minor (and won't affect perspective distortion), and for this discussion we're assuming ideal lenses.
The whole thing may seem counter-intuitive but it's true, it's just math.
Source:
https://en.wikipedia.org/wiki/Perspective_distortion_(photography)