Assertion: focal ratio | /r/DefinedAs | lens focal length
Facets:
value: in mm
type: OTHER
Source sentences:
text: The f-number, defined as the lens focal length in mm divided by the beam diameter at the lens in mm is at least 10, and preferably over 20.
text: The maximum usable aperture of a lens is specified as the focal ratio or f-number, defined as the lens's focal length divided by the effective aperture (or entrance pupil), a dimensionless number.