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The effect is achieved by using the setting of 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.
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, and the emotional impact of this effect is greater than the description above can suggest. 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.
Alternative names 
A dolly counter zoom is also variously known as:
- The "Hitchcock zoom" or the "Vertigo effect"
- "Hitchcock shot" or "Vertigo shot"
- Triple Reverse Zoom
- Reverse Tracking Shot
- Back Zoom Travelling
- Retrograde zoom
- "Smash Zoom" or "Smash Shot"
- Vertigo zoom
- A "Jaws shot"
- A "zido"
- A "zolly"
- Trombone shot
- The Trombone Effect
- A Stretch shot
- Reverse Pull
- More technically as forward zoom / reverse tracking or zoom in / dolly out
- Trans-trav (in Romanian), from trans-focal length operation and travelling movement
Purpose of the effect 
The dolly zoom is commonly used by filmmakers 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 filmmakers, 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).
Notable examples 
Spielberg used the technique again in E.T. the Extra-Terrestrial, Poltergeist, and Indiana Jones and the Last Crusade. The effect was also used in Michael Jackson's Thriller video, just as the zombies are gathering. It was originally used within the reimagined Battlestar Galactica to depict the feeling experienced by characters when the ship engages in faster-than-light travel. However, the technique was not used again until the fourth season.
A relatively slow and more subtle dolly zoom was also used in Martin Scorsese's 1990 film Goodfellas in the conversation scene between Henry Hill (Ray Liotta) and James 'Jimmy' Conway (Robert De Niro) set in a diner, with a view of the street forming the background of the shot.
The Lion King, an animated film, simulated a zoom shot in the scene where young Simba realizes the sound in the canyon is a wildebeest stampede. It is not a standard dolly zoom shot, as the "camera" zooms in on Simba, but the background does pull away dramatically, providing a similar effect.
In The Quick and the Dead, a dolly zoom is used to heighten the build-up in a series of pistol duels. For example, the duel between Herod (Gene Hackman) and The Kid (Leonardo DiCaprio) features a quick succession of dolly zooms as the contestants are about to draw their guns.
In The Lord of the Rings: The Fellowship of the Ring, the effect is used without a foreground subject. The purpose of the shot is to emphasize the sense of unreality and fear Frodo feels as the Nazgûl approach, on the road to Bree. The shot is used similarly, with no foreground subject, in The Lord of the Rings: The Return of the King, as Frodo is about to enter the cave of Shelob.
Many of the car reviews on the BBC television program Top Gear use a dolly zoom shot of the front of the car at speed.
For most purposes, we can assume the image space and the object space are in the same medium. Thus, for an object in focus, the distance between the lens and image plane , the distance between lens and the object , and the focal length are related by:
The transverse magnification is related by
The axial magnification of an object at is the rate of change of the lens-image distance as the lens-object distance changes. For an object of finite depth, one can conceive of the average axial magnification as the ratio of the depth of the image and the depth of the object:
One can see that if magnification remains constant, a longer focal length results in a smaller axial magnification, and a smaller focal length a larger axial magnification. That is, when using a longer focal length while moving the camera/lens away from the object to maintain the same magnification M, objects seem shallower, and the axial distances between objects seem shorter. The opposite-- increased axial magnification -- happens with shorter focal lengths while moving the camera/lens towards the object.
Calculating distances 
To achieve the effect the camera needs to be positioned at a certain distance from the object that is supposed to remain still during the dolly zoom. The distance depends on how wide the scene is to be filmed, and on the field of view (FOV) of the camera lens. Before calculating the distances needed at the different fields of view, the constant width of the scene has to be calculated. For example, a FOV of 90° and a distance of two meters yield a constant width of four meters, allowing a four-meter-wide object to remain still inside the frame during the effect.