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f-number, f number
Photog the numerical value of the relative aperture. If the relative aperture is f8, 8 is the f-number and indicates that the focal length of the lens is 8 times the size of the lens aperture
f number[′ef ‚nəm·bər]
A lens rating obtained by dividing the lens's focal length by its effective maximum diameter; the larger the f number, the less exposure is given. Also known as focal ratio. Also known as stop number.