The focal length of a lens is a measurement in millimetres from the lens to the camera's film or CCD and is a good indication of the magnification and angle of view. When using 35mm format cameras the standard lens is 50mm. It’s called standard because it gives roughly the same magnification as the human eye, which is actually around 43mm. As 35mm format has been the standard for so long all focal lengths for digital cameras are given with the equivalent in 35mm format. See the focal lengths comparison table here
Anything longer than a standard lens is classed as a telephoto lens and magnifies the subject. Super telephotos are lenses longer than 300mm. There are times when your budget may not allow the purchase of a super long telephoto. If so there are attachments made to extand the focal length. These teleconverters are available in 1.4x, 2x and 3x magnifications and fit between the camera body and lens.
Anything shorter than 50mm is classed as wide-angle and decreases magnification, but gains a wider angle-of-view to capture more of the scene in the frame.
Ultra wide-angles are lenses wider than 21mm.
There are also special lenses, such as macros, that allow superb close up facilities and fish-eye lenses that have a 180 degree angle-of-view and produce circular images.
In most cases the physical size of a lens is relative to the focal length, a 400mm lens will be longer than a 28mm lens for example. One exception to this is a mirror or reflex lens, so called because it has a set of mirrors inside the lens to fold the light, making the lens about half as long as a conventional telephoto.
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