Originally posted by SyncGuy Hi, i've just spotted this and i am thinking whether this is true.

I thought that a longer FL would "stretch" the background further, therefore larger, as compared to one from a shorter FL?

What happens is that the background and the foreground subject size will differ between the 2 sensor sizes due to the similarity in composition but with a change in camera distance, right?

I'm not quite sure I get what you're saying here, but let me try to explain the fundamentals of perspective in this example:

- You have two exactly identical statues, Statue 1 is placed 10 feet away, Statue 2 placed at 20 feet. In this arrangement, Statue 2 will appear to be half the size of Statue 1 even though they are actually the same size.

- Step back 10 feet. The Statue 2 will now be at 30 feet, and will appear to have shrunk by 33%. The Statue 1 is now at 20 feet and appears to have shrunk by 50%. Statue 2 now appears to be 2/3 the size of Statue 1, rather than the original 1/2. The further away you go, the closer the two statues will look in size (the ratio of apparent size approaches 1:1).

- Go back to start and do the reverse: step forward 5 feet. The Statue 1 is now at 5 feet, and appears to have increased in size by 100%. The Statue 2 is now at 15 feet and appears to have increased in size by 33%. Statue 1 now appears to be 3x the size of Statue 2, rather than the original 2x. The closer you get to Statue 1, the bigger it will look compared to Statue 2.

This is simple geometry, and is a universal concept. Focal length, sensor size and whatnot has no effect on any of this. You don't even need a camera, just see for yourself with your own eyes.

To prove that focal length has no effect on perspective, there's a simple test you can try: Put the camera on a tripod, use a 50mm lens and shoot a certain scene centered on a particular subject. Now do not move the camera, use a 100mm lens and shoot centered on the same subject. Do a 2x crop with the 50mm shot. Besides a difference in DOF, the two results will look exactly the same!

The so-called "telephoto/wide-angle perspective distortion" is nothing more than the result of you being at different distances to get the same primary subject size with each, further away with narrower angles ("telephoto"), and closer with wide angles. They look strange because it is hard to judge size and distance when the angle of view is very different from your own eyes' central focus area.