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. The zoom shifts from a wide-angle view into a more tighter-packed angle. In its classic form, the camera angle is pulled away from a subject while the lens zooms in, or vice versa. The dolly zoom's switch in lenses can help audiences identify the visual difference between wide-angle lenses and telephoto lenses. 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. Hence, the dolly zoom effect can be broken down into three main components: the moving direction of the camera, the dolly speed, and the camera lens' focal length.
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 conceived by Alfred Hitchcock during the filming of 1940's Rebecca, but he was unable to achieve the desired results. Some 18 years later, success came through Irmin Roberts, a Paramount second-unit cameraman, who devised the proper method for Hitchcock's film Vertigo. It is thought that Alfred Hitchcock specifically asked Roberts to assist him in creating a shot that exemplifies being in a drunk state after fainting at a party. At the time, Roberts had already designed a special camera capable of fast focal lens changes that allowed short-range projections. His expertise in focal lenses most likely prompted his innovation of the dolly zoom, which was more popularly recognized as the "trombone shot" or "contra zoom". Despite this step forward for cinematography, Roberts was not properly credited at the end of Vertigo. However, this 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). Director Joe Dante referred to it as the "Jaws Shot" since the scene in Jaws, when Roy Scheider sees the shark attack of the little boy Alex, is the most famous use of this shot.
Among the many creative uses the dolly zoom can provide to cinematographers, the shot can be divided into two types: the dolly-in/zoom-out and the dolly-out/zoom-in. The dolly-in/zoom-out shot is usually centered on a subject, where the background is pushed away from the character to create a profuse amount of uneasiness. For example, Poltergeist's infamous dolly zoom stretches the background to make it seem as if the door is much farther away from the character than it actually is. In contrast, the dolly-out/zoom-in shot shrinks the background to seem much closer than it really is.
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. In general, the dolly zoom's amplification of emotion is a special effect that compliments a director's arsenal of creativity. For example, in Sam Raimi's The Quick and the Dead, a dolly zoom, coupled with a Dutch angle shot, exemplifies drama between an intense shootout. An uneasy feeling of suspense can also be signified through a dolly zoom, most notably used in the movie Split in 2018, where Casey Cooke peers off into the distance in unwanted curiosity. Other uses include demonstrating overwhelming fear or important epiphanies for a character. The dolly zoom can also be utilized for the purposes of tonal shifts within the film. This was most exemplified in The Lord of the Rings: The Fellowship of the Ring, where Peter Jackson chained events together with more flare as opposed to a regular transition. Directors may also decide to use the dolly zoom as an alternative to the generic wide shot in order to give sufficient exposition on the upcoming scene.
In Goodfellas (1990), Scorsese uses dolly zooms to convey tensions between characters. This shot is most famously employed in Henry's dive into paranoia, where he eats at a diner with Jimmy while tracking a window to see if anybody has been following him.
In Ratatouille (2007), the food critic has an intense flashback, signified through a dolly zoom, towards his childhood days after eating Remy's ratatouille. Throughout the film, dolly zooms are used extensively to highlight the bonding between two characters, such as when Remy feels a personal connection with Chef Gusteau on television.
In Guardians of the Galaxy Vol. 2 (2017), Ego emotionally manipulates his son, Peter Quill, into helping him conquer the universe. However, upon confessing that he was the one who gave Peter's mother cancer, Peter immediately snaps out of his trance-like state and attacks his father. This realization is signified by a dolly zoom.
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 , the distance between lens and the object , and the focal length are related by
Then the transverse magnification is
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 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.
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.
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