Lens compression is a peculiar effect that occurs when you change the focal length of the lens.
There are two effects of changing your focal length (either by zooming in and out with your telephoto, and/or changing to a different lens size on your SLR camera).
The first effect is the potential distortion of your subject at a small focal length, such as at 16mm, or a flattering perspective at a larger focal length (such as at 200mm).
The second effect is the relative size of background objects in relation to your subject.
Let’s take a look at these two effects in an example.
When you look at these images, if you haven’t seen this effect before, it can be quite astonishing. There’s no trickery here – we didn’t move the trees or the model or the vehicles in the background. This is all done by changing the focal length of the lens and repositioning the camera either further or closer to the subject (the model).
In the first image, note how flattering the capture of the model is. There is no distortion of the appearance of her body.
Also note how close the black truck and the background tree appear to be in relation to the foreground tree and model.