Focal length isn’t a measure of how long or short a lens is physically, but the distance in millimetres from the optical centre of a lens to the imaging sensor when the lens is focused at infinity.
Rather than being fazed by the physics, it’s easier to think of the way in which focal length affects image size.
For a camera with a full-frame sensor, for example, a standard lens (one that gives a similar perspective to the human eye) is 50mm.
Lenses with focal lengths less than 50mm are referred to as wide-angles – simply because they have a wider angle of view. To make understanding focal length more difficult, the same focal length lens gives different views on cameras with various sensor sizes, because of the crop factor (the sensor only takes up part of the projected image). As a result, many manufactures give a 35 mm-format equivalent on lenses designed for cameras with smaller sensors and in this article descriptions are based on on 35 mm-format. Therefore, if your camera has a smaller sensor, and there's a good chance it does, you'll need to consider this when deciding which lens you need.
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