f/number

Definition: Setting of lens diaphragm that determines amount of light transmitted by lens. * Equal to focal length of lens divided by diameter of entrance pupil. * f/numbers are, for convenience and by convention, placed on a scale in which each standard f/number step (f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64 and so on) represents a doubling in the amount of light transmitted e.g. f/4 transmits twice as much light as f/5.6; conversely, f/16 transmits a quarter of f/8. * Since f/number is usually calculated from simple physical dimensions, different lens designs, varying focus and the use of accessories may all affect the actual amount of light projected: one lens set to e.g. f/8 may not give quite the same exposure as another lens set to f/8.
Assuming the ideal
The f/number of a lens is defined by simple geometry (a length divided by another) so it assumes that the lens passes all of the light entering it. But no lens does: each interface between media of different refractive indexes causes a loss of light. Modern lenses are amazingly efficient so losses are very small and, anyway, losses are automatically compensated by TTL metering.

Related Terms: T-number, transmittance

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