According to Wikipedia the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil. It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography.
The constriction of light through an aperture has nothing to do with its constant speed, however German scientists at the University of Darmstadt have claimed to have frozen light for a single minute to test its quantum coherence properties (i.e. its information state) in hopes of someday breaking the speed limit of light.
This phenomena naturally occurs in diamonds, in which light slows to half-speed.
Another reason why diamonds are a women's best friend.
They stop time for their beauty.
The Germans under their group leader George Heinze brought the speed of its light to zero, proving they are no constants in this universe.
Only variables of constancy, then again anyone who has lived with a woman knows that oh too well.