Controlling photographic exposure is to control the amount of light the camera sensor is exposed to. And the amount of light is determined by the aperature or diameter "d" of the opening. The lens diameter is controlled by the f stop which ultimate determines the maximum amount of light that can enter the lens. The diameter is equal to the focal length "f" divided by the f stop value. For example, the diameter is 36mm for a 50mm focal length lens with f stop of 1.4. Below are examples for several standard focal length lens.
| f Stop | 16mm Lens Dia. | 24mm Lens Dia. | 50mm Lens Dia. | 100mm Lens Dia. | 300mm Lens Dia. |
|---|---|---|---|---|---|
| 1.4 | 11 mm | 17.8 mm | 36 mm | 71 mm | 214 mm |
| 2.8 | 5.7 mm | 8.5 mm | 17.8 mm | 36 mm | 107 mm |
| 5.6 | 2.8 mm | 4.2 mm | 9 mm | 17.8 mm | 53.5 mm |
| 11 | 1.4 mm | 2.1 mm | 4.5 mm | 9 mm | 27 mm |
