The effect is achieved by zooming a zoom lens to adjust the angle of view (often referred to as field of view or FOV) while the camera dollies (or moves) towards or away from the subject in such a way as to keep the subject the same size in the frame throughout. In its classic form, the camera angle is pulled away from a subject while the lens zooms in, or vice-versa. Thus, during the zoom, there is a continuous perspective distortion, the most directly noticeable feature being that the background appears to change size relative to the subject.
The visual appearance for the viewer is that either the background suddenly grows in size and detail and overwhelms the foreground, or the foreground becomes immense and dominates its previous setting, depending on which way the dolly zoom is executed. As the human visual system uses both size and perspective cues to judge the relative sizes of objects, seeing a perspective change without a size change is a highly unsettling effect, often with strong emotional impact.
The effect was first developed by Irmin Roberts, a Paramount second-unit cameraman, and was famously used by Alfred Hitchcock in his film Vertigo.
The dolly zoom is commonly used by film makers to represent the sensation of vertigo, a "falling-away-from-oneself feeling" or a feeling of unreality, or to suggest that a character is undergoing a realization that causes him or her to reassess everything he or she had previously believed. After Hitchcock popularized the effect (he used it again for a climactic revelation in Marnie), the technique was used by many other film makers, and eventually became regarded as a gimmick or cliché. This was especially true after director Steven Spielberg repopularized the effect in his highly regarded film Jaws, in a memorable shot of a dolly zoom into Police Chief Brody's (Roy Scheider) stunned reaction at the climax of a shark attack on a beach after a suspenseful build-up.(see above)