Originally posted by dms Telephoto lens compress space because you move further away to get the same magnification, and thus relative distance (between nearer and farther away elements) is lessened. Similarly with wide angle lens you move closer and thus relative distances are increased. This effect has nothing to do with sensor size.

Originally posted by Cee Cee agreed

this has to do with how far away from your subject you are, you can test this without even using a camera - the closer you get to a subject the larger it becomes and hence the smaller the background detail relative to the subject, different focal length lenses put you at different distances from the subject if you maintain the same size of the main subject in the frame. When they talk about "equivalent" focal lengths on full frame & crop its more to do angle of view I think. The notion of "equivalent" focal lengths IMHO is a nonsense, a lens with the same field of view will give you a different perspective.

To make it absolutely clear, the "compression" is actually the result of the difference in magnification between two subjects of different distances. And the fact that when you aim to achieve the same principle image size, the secondary or further away image has a different magnification ratio when compared to the first, because with a shorter lens you move forward to retain the same magnification and change the perspective.

I will show this with the following 2 examples

1) Consider two people each 1.5 meters tall one standing behind the other by 1 meter shot with a 100mm lens from a distance of 10 meters.

Considering image size = subject size * focal length / distance. The two people will be 15mm and 13.64 mm respectively. Or the person in front appears 10* bigger.

If you use a 50mm lens and shoot from the same distance you will get images 1/2 the size but the ratio of sizes remains the same,

What this shows, is perspective is constant for all focal lengths providing the photo is taken from the same position

2) But now move forward with the 50mm lens so that the person in front is the same size on the image, to do this, you are shooting from 5 meters, but now, when considering the image size of the person 1 meter behind, the person 1 meter behind is now 12.5 mm and the person in front appears to be 20% bigger,

This is because when you move closer with the shorter lens to get a comparable image size,