A zoom lens uses a series of focal elements to produce a range of focal lengths. When attached to a motion picture camera, these elements can be adjusted to alter the focal length while the shot is in progress. The lens allows light to pass through the elements as well as the variable space between elements, resulting in a wider (wide angle) or more narrow (telephoto) image. Thus each frame captured on film during the zoom is actually shot at a slightly different focal length, resulting in the smooth motion in or out when the film is projected.
The technology behind zoom lenses was pioneered shortly after the invention of photography in the mid-19th century. Filmmakers likewise began using zooms soon after their medium appeared. Technological advances in the early- and mid-20th century made the zoom lens a ubiquitous piece of equipment for film productions.
Certain filmmakers have become associated with their frequent use of zoom effects. Robert Altman's films contain many notable zooms, often with the intention of positioning the viewer as a spectator within the scene. The zoom is Altman's way of guiding the eye of the viewer.
Stanley Kubrick is also associated with zoom effects. Perhaps most famously, Kubrick uses many zooms in his film "Barry Lyndon" to pull out from an image, revealing a dramatic composition by showing the surrounding space.
The uses of zoom effects can be broken down into two major categories: practical and aesthetic. Practical zooms are performed to draw attention to some specific element of a shot that is of great importance. In the classical Hollywood cinema, this often meant zooming in on a letter in order for the audience to be able to read its text on-screen. Filmmakers have also employed practical zooms to draw exaggerated attention to a detail that may appear to be minor but will figure importantly in the plot at some point in the future. Practical zoom effects might perform a number of other tasks, like picking a character out of a crowd by zooming in on his face.
Aesthetic zooms are those that lack a practical motivation. These zoom effects are frequent in films with a more visually conspicuous style, such as action films. Here the zoom might be used to find a more pleasing composition or fill the frame with motion. One important consideration for these aesthetic zooms is the possibility that the audience will be confused by the mobile camera if too much attention is drawn to its motion. While the point-of-view of a film camera generally precludes the presence of a camera operator, this illusion can be shattered by too much unmotivated zooming.
Aesthetic zoom effects also include zooms that are meant to imply a character's point of view. This may be a function of editing, as when a character's face is shown followed by a shot that includes a zoom in on an object. Often the result will be that viewers understand the zoom shot to be the character's visual point of view, and the zoom as emblematic of her psychological point of view as she focuses on the object.