f-stop

« BACK TO DICTIONARY INDEX

(n.) In photography, a number that represents a logarithmic value for the intensity of light in a given lens. It is a factor of the ratio between the focal distance and the diameter of the iris, or lens opening. A lens with a focal distance of 70 mm and a diaphragm diameter of 20 mm has an f-stop value of 7:2, which may be reduced to 3.5:1. This is expressed as an f-stop of f3.5. Lower ratios indicate larger lens openings and shorter focus depths. The f-stop is initially set for the depth of field. Each increasing stop doubles the amount of light emitted.