A T-stop is a measurement of the actual amount of light a lens transmits, accounting for absorption and internal reflection losses across all glass elements and coatings. The T number is a corrected, measured value derived from photometric testing, whereas the geometric f-stop is calculated purely from focal length divided by aperture diameter. A lens marked f/2.8 might only transmit T/3.1 worth of light because some photons are lost on the way through.
Cinema lenses are universally marked in T-stops because motion picture and video work requires precise, repeatable exposure consistency when cutting between lenses on the same scene. If a 35mm prime at f/2.8 transmits a stop differently from a 50mm prime at f/2.8, the cuts would visibly jump in brightness. T-stop marking forces every lens to be calibrated against a real light-transmission standard, so that switching between, say, an ARRI Master Prime 50mm at T/1.3 and a Zeiss CP.3 35mm at T/2.1 produces a match the colorist does not have to fix later.
Still photography lenses are marked in f-stops because depth of field, diffraction, and bokeh all depend on the geometric ratio rather than the actual light transmission. Two lenses at f/2.8 will produce the same depth of field at the same focal length and subject distance regardless of their T-stops. Exposure differences between lenses are typically small enough that the camera’s meter, or a quick check in manual mode, handles them transparently.
The gap between f-stop and T-stop is determined by the number of elements, the quality of anti-reflective coatings, and the absorption characteristics of the glass types used. Modern multi-coated lenses with 6 to 12 elements typically lose around a third to a half stop. Older uncoated lenses can lose a full stop or more, and exotic designs with many high-refractive-index elements lose more still. This is one reason vintage cinema lenses, despite their character, sometimes test slower than their geometric apertures suggest.
For hybrid shooters who use the same lenses for stills and video, the practical rule is to think in T-stops on motion work and in f-stops on stills. Switching lenses on a video shoot without a matched T-stop will produce inconsistent exposure, especially noticeable on faces. Photographers who buy still lenses for cinema work should expect to chart each lens against a gray card under controlled light, or invest in dedicated cine-style primes where the T-stop is engraved on the barrel.