F stops and T stops

Guy Meachin · July 28, 2005

What is the difference between F stops and T stops?

Guy

Laurent Andrieux · July 28, 2005

F stops (N number) are defined geometrically, from the focal length (f) and the enter pupil diameter (D) of the lens : N=f/d. It does not take account of the fact that the lens is not purely theorical, and that for diferent reasons, there is a photometric transfer function as light passes through the lens, at the same time as a geometrical construction.

This photometric transfer depends on different factors, such as the number of lenses in the optical block, their anti-flare treatment, the "quality of the glass etc.

Therefore, manufacturers engrave the value that gives the photometrical aperture better than the real geometrical aperture, so that an operator can set the lens at a given stop being sure the quantity of light is the same, whatever lens it is, even though the real geometrical aperture obtained is not the one engraved.

Usually, new lenses are looseless than older ones, prime lenses are looseless than zooms... (just because they "cut " less light).

This is the T stop, for Transfer. Nobody really cares, it's just that when it comes to calculating depth of field, based on purely geometrical things, the stop one should use for the calculation is not the one that is engraved (the geometrical F stop is ovbiously wider than the photometric T-stop). This is why you find a "fix lens" and a "zoom lens" value on calculators like samcine mark II calculators...

Usually this difference is less than a third of a stop, though it can go up to 2 thirds on old zoom lenses.