35mm equivalent
The 35mm equivalent focal length of a lens is sometimes given for cameras using other film formats, and often for digital cameras. This is to give some comparison of the angle of view of lenses between different cameras with different film frame or digital sensor sizes.
Typical 35mm-format lenses standard lenses are between (approximately) 40-55mm. Telephoto lenses being longer, and wide angle being shorter.
The equivalent is calculated by the dividing standard full-frame 35mm diagonal, of 43.27mm, by the diagonal of the camera's sensor/film frame diagonal, and multiplying the lens focal length by this ratio. Thus, for example, an APS camera with a typical lens of, say 25mm, using the full HDTV format (30.2 x 16.7 mm), has a frame diagonal of 34.51mm, so the lens' 35mm equivalent is 25*43.27/34.51 = 31mm.
