f Ratio or f Number or f Stop (f/ or f)
f Ratio or f Number or f Stop (f/ or f) :
A measure of the light-gathering power of the lenses in cameras and telescopes. The f ratio, for example "f/4" or "f4," is the aperture of the lens (the effective diameter of the lens, which may be reduced or "stopped down" for the exposure; see also stop) expressed as a fraction of the focal length of the lens (the distance from the lens to the point where light is focused). Thus "f/4" indicates that the aperture is 1/4 the focal length. In cameras the f ratio is proportional to the square root of the exposure time, so an f/8 setting requires a four times ((8/4)2) longer exposure than an f/4 setting. Because of this connection with exposure times, the f ratio is often said to express the "speed" of a lens
No records Found