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
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.
I can explain, but it’s better you see a picture. A lens is just a small part of a sphere, what matters is the curvature. Doesn’t matter how small a piece you take, it’ll always bear the properties of the sphere it comes from (at least focal length). Here is a picture: https://images.app.goo.gl/fR9GLw3rvZjQWDVSA
Because those camera lenses are not a simple, single lens. They are instead a number of different lenses, arranged to product the effect of a longer lens.
As a simple example, you can make a concave lens, one that would scatter instead of focusing light, and put it between your main lens and your film plane, This creates the effect of a longer lens, and how long a lens depends on where you place it – this is the basic idea of a ‘zoom’ lens with an effective focal length you can adjust.
In addition to the other explanations, it’s also possible to have [catadioptric “lenses”](https://en.wikipedia.org/wiki/Catadioptric_system#Photographic_catadioptric_lenses) which use mirrors instead of lens elements. Because the light travels backwards and forwards three times along the length of the “lens,” it can be much shorter than its effective focal length.
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