Normally an iris diaphragm is used to vary the aperture setting, the settings are called f stops. Move the mouse over the f settings below and see the aperture change. The f number scale was originally devised because the image brightness not only depends upon the size of the aperture but also the focal length of the lens. The lens and aperture are both taken into consideration by the lens manufacturer to determine the f stop settings of the lens. The f stop settings are universal, for example f 8 on different lenses will produce images of the same brightness.
Photographic exposure is determined by the image brightness and exposure time.
Increased exposure time can compensate for a dim image. Exposure time, often measured in fractions of a second, may also determine the sharpness of the picture. A long exposure will record movement as blurs and it is possible for high speed photography to freeze a speeding bullet. For any photographic image there is only one point of true focus, in front of and behind this point there is a zone where the image is acceptably sharp. The Depth of Field varies depending upon the aperture and focal length of the lens.