Dolly zoom
A dolly zoom (also known as a Hitchcock shot, "Vertigo" shot, "Jaws" effect, or Zolly shot) is an in-camera effect that appears to undermine normal visual perception. 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 (moves) toward 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. Background. The effect was first conceived by Irmin Roberts, a Paramount second-unit cameraman, in Alfred Hitchcock's film "Vertigo". The shot has since been used in many other films, including "Goodfellas", "Jaws", and the "Lord of the Rings" films. Rainer Werner Fassbinder uses the effect twice in one shot in "Chinese Roulette" (1976). 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 them to reassess everything  they had previously believed. Alfred Hitchcock popularized the effect, and he used it for a climactic revelation in "Vertigo". Optics. For most purposes, it can be assumed that 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 formula_1, the distance between lens and the object formula_2, and the focal length formula_3 are related by Then the transverse magnification is The "axial magnification" formula_6 of an object at formula_2 is the rate of change of the lens–image distance formula_1 as the lens–object  distance formula_2 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 in 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 2 meters yield a  constant width of 4 meters, allowing a 4-meter-wide object to remain still inside the frame during the effect.