An f-stop is a purely geometric ratio between a lens’s focal length and its effective aperture diameter, while a T-stop is a measured value that adjusts that geometric number for the actual light lost to absorption and reflection inside the real glass and coatings, so two lenses sharing the same f-number can still expose a scene differently, while two lenses sharing the same T-number should produce matching exposure regardless of their geometric aperture rating. A T-stop only describes light transmission, not depth of field, so the two markings answer different questions and should not be substituted for each other. Knowing the difference tells you when to trust an f-stop for controlling depth of field and when you need a T-stop for matching exposure across multiple lenses.
Two Numbers With Different Jobs
The f-number is calculated directly from a lens’s design geometry: focal length divided by the effective diameter of the entrance pupil, with no reference at all to how much of the light entering the front element actually reaches the sensor. It describes the light-gathering cone’s geometry, useful for both exposure and depth-of-field calculations in a theoretically perfect, lossless lens.
The T-number, or T-stop, starts from that same geometric f-number but is then calibrated on a bench for the specific optical design’s real transmission efficiency, accounting for light absorbed within the glass elements, lost to internal reflections at each air-to-glass surface, and reduced by the quality of the anti-reflective coatings used. A T-stop therefore describes actual light throughput reaching the image plane, which is what determines real exposure, rather than pure geometry.
Match Exposure Across Lenses
Because element count, glass composition and coating technology differ between lens designs, two lenses both marked at the same f-number do not necessarily transmit an identical amount of light. A lens built with more elements or older, less effective coatings can lose meaningfully more light internally than a simpler or better-coated design at the same nominal f-number, sometimes by a noticeable fraction of a stop or more. This becomes important whenever exposure needs to match precisely across several different lenses, such as cutting between shots from different lenses on the same production, which is a major reason cinema lens sets are calibrated and marked in T-stops: every lens in a matched set is measured so the same T-number produces the same actual exposure, regardless of small transmission differences in the underlying glass.
Keep Depth of Field Separate
Depth of field is governed by the geometric aperture diameter, meaning the f-number, together with focal length, focus distance and sensor or format size, not by how much light is lost to internal transmission. A T-stop cannot be substituted into a depth-of-field calculation. Two lenses set to match the same T-number but built with different transmission efficiencies will have different underlying f-numbers, and therefore different depth of field at that matched T-stop, even though their exposure will match exactly. Matching T-stops guarantees matching exposure; it does not guarantee matching background blur or depth of field between two different lenses.
Read Cinema and Photo Markings
Cinema lenses are commonly engraved with T-stops, sometimes alongside a smaller reference f-number, reflecting their design purpose of predictable, repeatable exposure when multiple lenses are used across the same production and need to match seamlessly on a waveform monitor or light meter. Most stills photography lenses, on the other hand, are marked only with f-stops and have no published T-stop value at all, since through-the-lens metering in a stills camera measures and compensates for actual light reaching the sensor automatically, making a separately published transmission figure less essential for that workflow. It should not be assumed that every photographic lens has a documented T-number simply because the concept exists; check the specific lens’s own published specifications rather than assuming one is available.