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Due to the popularity of the 35 mm standard, camera–lens combinations
are often described in terms of their 35 mm equivalent focal length,
that is, the focal length of a lens that would have the same angle of
view, or field of view, if used on a full-frame 35 mm camera. Use of a
35 mm-equivalent focal length is particularly common with digital
cameras, which often use sensors smaller than 35 mm film, and so
require correspondingly shorter focal lengths to achieve a given angle
of view, by a factor known as the crop factor.