If the focal length of a camera lens is the distance between the optical centre of the lens and the camera sensor, how is a 400mm lens less than 400mm long?

Question

If the focal length of a camera lens is the distance between the optical centre of the lens and the camera sensor, how is a 400mm lens less than 400mm long?

In: Technology

Answer 1

Lens focal lengths are often given as the [35 mm equivalent](https://en.wikipedia.org/wiki/35_mm_equivalent_focal_length), meaning that the lens’s angle of view is the same as the angle you would get with a 400mm lens on a 35mm camera. Most cameras nowadays use smaller sensors than 35mm, so the actual focal length needed to achieve an equivalent 400mm focal length is shorter.