There are a lot of different terms for the idea that the focal length of the lens you use for a given photo affects the perspective of the photo. Probably the two most common are “distance compression” and “compressed perspective”. No matter what term you use to describe the effect however, the idea that it's due to the focal length of your lens, it isn't entirely true.
In reality, there's another factor involved in making a background appear closer to or farther from your subject, and it's really very simple to prove. If you'd like to see it for yourself, give this a try:
- Grab your favorite zoom lens and a (very patient) model or a prop to use as a subject. (You'll probably want your camera, too.)
- Head out to your favorite outdoor location where you can put a little distance between your subject and the background.
- Set your camera to your favorite auto-exposure mode. (Remember, this is just an experiment.)
- Set up your prop or pose your model at a reasonable distance from the background and step back to a distance from your subject that lets you get a good chunk of the background in the shot at the highest focal length setting on your lens. Center your subject in the frame and take the shot.
- Stay where you are, and adjust your lens to its lowest focal length. Center your subject again and take another shot.
- Take those shots back home and download them to your computer.
- Open both images in any editing program.
- Crop the shot taken at the shorter focal length so that the amount of background included in the shot is as close as possible to what's in the longer focal length shot.
- Compare the 2 images at the same zoom ratio to see if the background appears closer to the subject in either shot. (if possible, drop the cropped image into a layer on top of the uncropped one and resize it to match.)
What you'll find, if you've followed the instructions above accurately, is that there's virtually no difference in the apparent distance between the background and the model. Myth dispelled. Well, sort of.
(Success Tip #1: Learn to create better photos with this simple deck of cards.)
If you go to the trouble to set up the same shoot and move forward on the second shot to frame your subject at the same size in your viewfinder or LCD as in the first shot, you'll see a completely different result back at home. In fact, you should be able to notice it in the field.
The explanation is simple. The only thing that changes when you adjust the focal length of your lens on your camera is the field of view. That's why moving closer to or farther from your subject as you change focal lengths will change the apparent distance between the subject and background. When you add that factor, you can use a change in focal length very effectively to determine the impact the background will have on your final image.
(Success Tip #2: Shoot people anywhere and get paid - easily.)
Now, in case you didn't jump right up and try the experiment laid out above, I'm going to let Adorama's Gavin Hoey demonstrate the whole process, from start to finish, in the video below. He does a great job of showing you exactly what I've described and he actually goes through the experiment both ways. He also explains very clearly exactly how you can use the effects to your advantage. Take a look: