A system for designating lens apertures, usually referred to as “T-stops.” T- stops are mostly used on lenses for professional cinematography, because in a motion picture the exposures of shots taken with different lenses must match exactly, so that the shots will cut together smoothly.
At any given f-stop, less light is transmitted through a lens with many elements, such as a zoom or extreme wide angle lens, than would be transmitted through a simpler lens. Instead of being calculated geometrically as f-stops are, T-stops are determined by actually measuring the amount of light transmitted through the lens (the “T” stands for transmission). The numbers and the relationships between them are the same as for f-stops. There is no fixed relationship between f-stops and T-stops.
Calculations involving lens geometry, such as depth of field, field of view and so on, must be done with f-stops, not T-stops.
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Last revised: 25 October 2001.