Exposure and Metering Modes
Exposure is determined by the amount of light, the sensitivity of the film or electronic chip, lens aperture, and shutter speed, which must be adjusted so that the proper amount of light reaches the image plane in the camera.
The aperture ring opens and closes the diaphragm built into each lens. The size of the diaphragm opening is engraved on the lens in f stops or f numbers. A large opening, letting more light onto the image plane, is indicated by a small number, such as f/2.8. A small opening letting less light through the lens is a high number, such as f/16. On some lenses, especially those made in Europe, the aperture is engraved as a ratio; 1:4 means f/4. The f number is the ratio between ...