Editing Normal lens (section)

== The image circle ==
A normal lens typically has an angle of view that is close to one [[radian]] (~57.296˚) of the optical system's [[image circle]].{{citation needed|date=July 2018}} For [[135 film|135]] format (24 x 36&nbsp;mm), with an [[Incircle and excircles of a triangle|escribed]] image circle diameter equal to the diagonal of the frame (43.266&nbsp;mm), the focal length that has an angle of one radian of the inscribed circle is 39.6&nbsp;mm; the focal length that has an angle of one radian of the horizontally-bound [[Incircle and excircles of a triangle|inscribed]] image circle, is 33&nbsp;mm; the focal length that has an angle of one radian of the vertically-bound inscribed circle, is 22&nbsp;mm. This correlates with the popularity of 35 and 24&nbsp;mm lenses, and the existence of 40&nbsp;mm lenses, albeit the latter in a more restrained offer. A 50&nbsp;mm lens has a vertical-bound inscribed circle angle of view of ~0.5 radians. A 70&nbsp;mm focal length has a horizontally-bound inscribed circle angle of view of ~0.5 radians. An 85&nbsp;mm lens has an enscribed (frame diagonal) circle angle of view of ~0.5 radians. Effectively, the 24, 35 and 40&nbsp;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&nbsp;mm lenses closer to one radian than 50&nbsp;mm lenses.