Definition of f–number
1 : the ratio of the focal length to the aperture in an optical system
2 : a number following the symbol f/ that expresses the effectiveness of the aperture of a camera lens in relation to brightness of image so that the smaller the number the brighter the image and therefore the shorter the exposure required
Origin of f–number
First Known Use: circa 1903
Seen and Heard
What made you want to look up f–number? Please tell us where you read or heard it (including the quote, if possible).