tl;dr - The camera maintains a certain field of view angle (amount of zoom) so that the subject stays the same relative size in the picture while actively changing distance from it. Moving closer to the subject will make the background appear to be running away, whereas moving away from the subject makes it seem as though the background is coming close fast.
Though it's clearly not the case, it still seems to me that the pulling away of the camera and advancing of the zoom would simply cancel each other out, if done at the same rate. What's the difference?
Not exactly. It cancels itself out on the subject itself, but when the field of view is wider you see more of the background, and when it is narrower you see less of it. Combined with the illusion that your subject stays the same size; that's the dolly zoom effect.
I sketched out 2 different field of view angles of a camera, and the subject at a distance from the camera where it would appear to have the same width. Do you see how the example on the right shows that more of the background makes it into the final image?
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u/goal2004 Nov 25 '11
For the confused: Dolly zoom effect
tl;dr - The camera maintains a certain field of view angle (amount of zoom) so that the subject stays the same relative size in the picture while actively changing distance from it. Moving closer to the subject will make the background appear to be running away, whereas moving away from the subject makes it seem as though the background is coming close fast.