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Change the Focal Length of the lens during a shot. Looks like, but not exactly like, a dolly.

Usage: "Zoom in", "Zoom out".

Old-school TV cameras had a set of three lenses, mounted microscope style, or an interchange system like that of a film camera. Each had a specific, fixed focal length. Changes had to be conducted while the camera was off-air.

The zoom, or variable focal length, lens was a huge step forward. With the addition of a variable speed motor drive for the zoom, you can even adjust it during a shot with a single finger. Not only that, but any focal length within the designed range of the lens was possible. Thus one lens replaces a potentially infinite number of others.note 

The lenses of studio cameras have a focal length range that favors the close-in setting of their environment. A field camera has the ability to zoom in much tighter, but has a longer minimum distance from its target.

Cinematographers, the kind that shoot on actual film, don't like the zoom lens. The increased amount of glass the light must pass through in order to reach the film causes distortion and increases the chance for Lens Flare and other forms of glare. Also, the extensive planning that goes into each shot limits the need for on-the-fly focal length adjustments. The tube/CCD in a video camera has less resolution and contrast sensitivity than film, and thus is less vulnerable to these complaints, but over-use of zoom (particularly if the zoomed shot is not in focus, or if the zoom is jerky rather than smooth) is now fairly firmly ingrained in Western audiences as visual shorthand for "amateurish".

Consider also the Zoom Establishing Shot, where a camera starts off showing a building (and the sign in front telling you what the building is) and then tilts, pans and zooms in on a window to indicate that the next shot you see is totally taking place in that room, and not on a set on a soundstage in a completely different city. Popular in the Seventies, it became something of a Dead Horse Trope once anyone with home video gear could do it.

Combine it with a simultaneous Dolly for the Vertigo Effect. Compare the Staggered Zoom, when the zoom is accomplished with a series of cuts rather than continually changing the focal length, producing a much more startling effect.