Taken with a lens that has a variable focal length, a zoom shot is one that permits the cinematographer to change the lens’ focal length – and thus the apparent size of the subject within the frame – without moving the camera. In one continuous movement, this camera technique makes an object or subject in the frame appear larger or smaller.
Generally speaking, zoom lenses are an assembly of moving glass elements that equate to a particular focal length depending on a particular dialed in mechanical position. Complicated zoom lenses can have upwards of 30 moving elements which must match precisely in order to resolve an image. As a result, zoom lenses tend to be far more complicated than single focal length lenses (so-called prime lenses) and tend to have a lower maximum aperture – they let less light in, and are called slower lenses. Typically zoom lenses are faster when fully zoomed out, and the maximum aperture varies as the lens zooms in.
The longer the range of a zoom lens, the larger the compromise in speed or sharpness. As an example, the Sigma Art Series 18-35mm F1.8 DC HSM zoom lens covers a relatively narrow and wide focal range, but is very fast (1.8). Lenses that cover a wider focal length will by necessity be slower – their barrel length and number of elements mean that less light can travel through them. The Canon L series 24-70mm F2.8 II USM lens covers a larger focal length range, but is 1 and 1/3rd stops slower than the Sigma while also being less sharp. As the range increases, the compromises in speed and sharpness must also increase as a result in the number of elements and general size of the lens.
There are two different types of zoom lenses: parfocal and varifocal. Parfocal lenses use highly precise mechanics to maintain focus on a subject as the focal length changes. Varifocal lenses, on the other hand, change focus as they zoom in and out. Essentially all consumer grade zoom lenses are varifocal.
William Wellman’s 1927 film Wings (which won the first Oscar), employed the zoom lens, as did Paramount films throughout the 1920s and 30s. Prior to that, zoom lenses were relegated to news reels, and although zoom lenses have existed since the 19th century, they did not become precise enough to be commonly used in cinema until after WWII, when the huge investment in aerial photography enhanced the available optics in the movies of the 1950s. In 1962, Angénieux was awarded a Scientific and Technical Academy Award for the 12-120mm Super-16 zoom lens. In the late 60s and early 70s, Mario Bava made extensive use of the zoom lens.
Today, zooms are sometimes considered jarring due to the fact that they behave in a way in which the human eye cannot emulate. However, the zoom may suggest what the mind’s eye does, parallel to the iris matte effect. Here in Taxi Driver (1976), Travis’ attention telescopes to Betsy:
CLIP: (clip from Taxi Driver with zoom to Betsy’s desk thrown from Travis’s POV)
A related effect, the dolly-zoom or “Zolly” camera move, can be seen in Alfred Hitchcock’s Vertigo (1958), where the length of the stairway is made to seem longer with a combination of zooming in and dollying back.