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.
Note in this next image, we reduce the focal length to 70mm, and the black truck seems to get farther away.
The perspective on the model is still pleasing, with an unnoticeable perspective change.
The next two images you will see the black truck seem to get farther away, and subtly, the perspective on the model becomes less flattering.
In this image, the black truck and farthest tree are almost too small to see.
The model’s appearance is distorted, and could be interpreted as unflattering (or creative). Had we taken a higher or lower position for the camera, it would have been even more distorted (making her legs or head look bigger, depending on the camera height).
So as you can see, the focal length of your lens not only controls how far away you can shoot (how much you can zoom), but can also impact the appearance of the perspective of the image. This could be used for creative benefit, but can also be used at the subject / client’s expense.
Use this information wisely!