a radian is a measure of an angle that is approximately equal to 57.3 degrees.
a radian is a measure of an angle that is approximately equal to 57.3 degrees.
a radian is a measure of an angle that is approximately equal to?
57.3 degrees
radian









































the radian is the si unit for measuring angles, and is the standard unit of angular measure used in many areas of mathematics. the length of an arc of a unit circle is numerically equal to the measurement in radians of the angle that it subtends; one radian is just under 57.3 degrees (expansion at ). the unit was formerly an si supplementary unit, but this category was abolished in 1995 and the radian is now considered an si derived unit.









































separately, the si unit of solid angle measurement is the steradian.
radian
a normal lens typically has an angle of view that is close to one radian (~57.296˚) of the optical system's image circle. for 135 format (24 x 36 mm), with an escribed image circle diameter equal to the diagonal of the frame (43.266 mm), the focal length that has an angle of one radian of the escribed circle is 39.6 mm; the focal length that has an angle of one radian of the horizontally-bound inscribed image circle, is 33 mm; the focal length that has an angle of one radian of the vertically-bound inscribed circle, is 22 mm. this correlates with the popularity of 35 and 24 mmlenses, and the existence of 40mm lenses, albeit the latter in a more restrained offer. a 50 mm lens has a vertical-bound inscribed circle angle of view of ~0.5 radians. a 70mm focal length (typically only available in zoom lenses) has a horizontally-bound inscribed circle angle of view of ~0.5 radians. an 85 mm lens has an escribed (frame diagonal) circle angle of view of ~0.5 radians. effectively, the 24, 35 and 40 mm trio have a 1:2 relation to the 50, 70 and 85 trio of focal lengths. "normal" lenses, those that cover one radian in at least one of their inscribed or escribed image circles, belong to the first group, with 35 and 40 mm lenses closer to one radian than 50mm lenses.
normal lens