f number

f number

Designation for a photographic lens which gives the ratio of the focal length to the diameter of the effective aperture or entrance pupil. Example: f/8 means that the lens has a focal length eight times the diameter of the entrance pupil. Syn. f/stop; f-value; focal ratio; lens speed. See relative aperture.

Table F1 Sequence of f numbers used in photography with the corresponding relative image brightness and exposure time to maintain constant film exposure. The relative image brightness is equal to the square of the f number fraction
f number		focal length/pupil exact f number		diameter		relative image brightness		relative exposure time
f/0.5		0.5		f/0.500		1/0.25		0.12
f/0.7		0.7		f/0.707		1/0.5		0.25
f/1		1		f/1.000		1/1		0.5
f/1.4		1.4		f/1.414		1/2		1
f/2		2		f/2.000		1/4		2
f/2.8		2.8		f/2.828		1/8		4
f/4		4		f/4.000		1/16		8
f/5.6		5.6		f/5.657		1/32		16
f/8		8		f/8.000		1/64		32
f/11		11		f/11.314		1/128		64
f/16		16		f/16.000		1/256		128
f/22		22		f/22.627		1/512		256
f/32		32		f/32.000		1/1024		512