Today’s Question: I believe it is misleading to imply that only changes in actual distance (location of the camera relative to the items being shot) affects compression. The visual perception of compression can be achieved by either changing actual distance between objects (as you correctly stated) or by simulating that effect by using a longer focal lens, and to a lesser degree in post-processing. In my experience, cropping an image using software after an image has been captured in the camera can provide a more compressed view, though it does not give the exact same visual impact of capturing an image with a much longer focal length.
Tim’s Quick Answer: The phenomenon we refer to as “lens compression” is absolutely caused by changing your distance to the scene you are photographing, not by lens focal length or cropping in post-production.
More Detail: Moving your physical position relative to the scene you are photographing is what changes the perspective effect we refer to as “compression”. The focal length is simply adjusting the cropping of the scene. Part of the reason I think it is so critical to understand this is that it places an emphasis on the fact that photographers can change their position to alter the perspective they are capturing in their photographs.
Using a longer focal length lens while photographing from the same position will simply crop the scene. It will not create the effect of compression. To compress the scene you must move farther away from that scene. That distance creates the compression effect. Using a longer lens to retain the same framing of the scene is simply cropping the scene, not creating a compression effect.
You can see visual examples that prove this issue (hopefully once and for all) in a post on the GreyLearning blog here: