Pronunciation: ' ef- ˌ n ə m-b ə r
Function: noun
Etymology: f ocal length
Date: circa 1903
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