Some lenses, mainly those used in motion cinematography and not still photography, have apertures rated in T-stops. T-stops indicate the actual or absolute amount of light being passed through (transmitted) the aperture diaphragm. They differ therefore from f-stops, which indicate the aperture size relative to the focal length. (in a sense, T-stops are absolute and f-stops are relative)
T-stops can be an important concept with certain zoom lenses in particular, since zooms tend to result in some light loss, though this is not a huge issue for most modern multicoated lenses. Lenses with T-stop markings usually have the f-stops marked in white and the T-stops in red.
cf. aperture, f stop, transmittance.
Entry last updated 2002-05-09. Term 1253 of 1487.
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