Photographic lensA photographic lens (or more correctly, objective) is a lens used for a camera. 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 focussing 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.