Zoom lens.
A lens with a variable focal length is a true optical zoom.
Such lenses have a rotating or push-pull ring that lets you set the exact focal length you want to use. And altering the focal length of the lens alters the field of view. 28-70mm, 70-200mm and 28-200mm are common zoom lens specifications.
However, zoom lenses always involve design compromises over prime (fixed focal length) lenses. It’s much more complicated to build a zoom lens, and so high-quality zooms tend to be very expensive and low-quality zooms tend to offer poor optical quality. Cheap zooms also tend to be slow - very limited maximum apertures. And the greater the zoom range the worse quality tends to get.
There are some exceptions, of course. For example, Canon build a 35-350 lens which offers good optical quality despite its massive 10x zoom range. But the compromise comes in terms of size (the 35-350 is huge) and cost (it’s really expensive) and speed (the maximum aperture is relatively small, going from f/3.5 to f/5.6). Zoom lenses also have either fixed maximum apertures (constant aperture) or they have maximum apertures which vary according to the current focal length setting (variable aperture).
Note that some digital cameras advertise “digital zoom” which is a cheap way of faking zoom effects through digital interpolation. It offers low quality results and isn’t worth much - you’re better off cropping a picture and scaling it in an image editing program.
cf. constant aperture zoom lens, digital zoom, focal length, optical zoom, prime lens, push-pull, two touch, variable aperture zoom lens.
Entry last updated 2002-05-02. Term 1327 of 1487.
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