35mm equivalent

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The 35mm equivalent focal length of a lens is sometimes given for cameras that do not use 35mm film, most often for digital cameras. This is an attempt to relate the angle of coverage of various lenses to a familiar reference standard, when comparing cameras using film or sensor image formats of different sizes.

Typical 35mm-format standard or "normal" lenses are between 40-55mm. Telephoto focal lengths are longer, and wide-angles shorter than this. Thus a digital point & shoot might be described as having a 28-140mm (equivalent) lens, covering a full range from wide-angle to telephoto angles of view. However the true, optical focal length of such a lens might be some unfamiliar range such as 6.1–30.5 mm.

The equivalent is calculated by dividing the diagonal of the standard 35mm frame, 43.3 mm, by the diagonal of the camera's sensor/film frame diagonal, and multiplying the true lens focal length by this ratio. Thus, for example, an APS camera set to the wide image format (30.2 x 16.7 mm), has a frame diagonal of 34.5 mm; so a lens of 25mm focal length has a 35mm equivalent of 43.3 / 34.5 × 25 = 31 mm.

The use of "equivalent" focal lengths often creates ambiguity between a lens's actual focal length and the (fictional) equivalent one; expressing lens coverage in degrees (etc.) might have been preferable. Yet today the "35mm equivalent" convention seems so strongly entrenched that it may be difficult to displace.

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