Difference Between Lens Speed And Shutter
In addition to focal length, a lens is described by its speed. The ability of a lens to gather light reflected from a subject and thus to produce a sufficiently bright image on the film is measured by the f-number of the lens. The f-number is the ratio of the focal length of the lens to the lens diameter. A lens with high ability to collect light has a low f-number, such as f1.4, and is known as a fast lens. It can be used to photograph subjects that are in dim light. A lens with a high f-number, such as f8, has limited ability to collect light and can be used only when enough light is available.
While the lens aperture controls the intensity of light entering the camera, the shutter controls the length of time that light enters. Often the shutter control of the camera is marked only with the denominator of these fractions—1, 2, 4, 8, and so on.
Shutter speeds also may be used to produce photographic effects. A very low shutter speed shows motion through space, that is, the path of the motion. This means that the moving object will appear blurred if a slow shutter speed is used. For example, a 1/2- to 1-second shutter speed will show the path of movement of tall, blowing grass or swaying branches.
The faster the shutter speed, the less blur in the finished photograph. A very fast shutter speed “freezes” motion and shows the moving object more sharply.