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Each lens has numbers inscribed on the filter ring that give us important information about how it will perform. The information will read something like 1:1.8 f=45mm
The number specified in mm is the focal length; the distance between the optical center of the lens and the film of the camera when the lens is focused at infinity. This gives us an idea of the angle of view (AOV) as shorter lengths give wider angles. Here the focal length is 45mm.
Lenses with a focal length of 50mm are called “normal” because they create images with roughly the same perspective and size relationship of objects that we see with our eyes.
The other set of numbers refer to the maximum aperture of the lens. 1:1.8 indicates that the maximum aperture is f1.8. This is useful as we can tell how fast the lens is.