A 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:
Some lenses can change their focal length. This is called zooming, and lenses capable of this are zoom lenses. Lenses with fixed focal length are called prime lenses.
Some notable photographic optical lens designs are:
The lens may usually be focused by adjusting the distance from the lens assembly to the image-forming surface, or by moving elements within the lens assembly.
The maximum usable aperture of a lens is usually specified, as the focal ratio or f-number, the focal length divided by the actual aperture in the same units.
An extreme wideangle lens of large aperture must be of very complex construction to correct for optical aberrations, which are worse at the edge of the field and when the edge of a large lens is used for image-forming.
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