Photographic lensA photographic lens (or more correctly, objective) is an integrated system comprising one or more simple optical lens elements, used for a camera or microscope. It may be fixed to the camera body, or interchangeable. There will usually be an aperture selection mechanism for adjusting the amount of light through the lens, and a focusing mechanism. Depending on type of camera, there may also be an integrated shutter. The maximum aperture opening will normally be specified, as the f-number. The lower the number, the more light is admitted through the lens. On lenses for SLR cameras, there is usually an auto mechanism, where by the aperture is held fully open while focusing, but closes to its preset value when an image is taken. The most important characteristic is the focal length, which is usually measured in mm (cm up to around 1960), and which places the lens in one of three categories: Depending on whether the lens can or can not zoom, the photographic lens is also categorized as either a zoom lens or prime lens -- the latter have a fixed focal length as opposed to the former in which that parameter can be changed in order to achieve a zoom in/zoom out effect. Some notable photographic optical lens designs are:
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