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 measurd in mm (cm up to around 1960), and which places the lens in one of three categories:
Some notable photographic optical lens designs are: