What is a T-stop, and how does it differ from an f-stop?
Asked 3/31/2012
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I understand that f-stops/f-numbers describe aperture, but I also see T-stops mentioned, especially for cinema and video lenses. What does a T-stop measure, how is it related to the f-number, and why would someone use T-stops instead of f-stops?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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F-stops are purely geometrical, the ratio of aperture to focal length, regardless of actual light transmitted. But all lenses absorb a part of the light passing through them, and the amount being absorbed varies lens to lens. So, in situations where even the slightest change of lights being transmitted affect the output, i.e cinematography, where many images are seen in rapid succession and even small changes in exposure will be noticeable, T-Stop is used as an standard. Since all lenses absorb some light, the T-number of any given aperture on a lens will always be greater (less light transmission) than the f-number. For example, a lens with f-stop 2.8 can have a t-stop 3.2, meaning a small portion (about a quarter) of the transmitted light has been absorbed by the lens glass elements.
A real lens set to a particular T-stop will, by definition, transmit the same amount of light as an ideal lens with 100% transmission at the corresponding f-stop. A f/2.8 lens can have t/3.2 and another f/2.8 lens can have t/3.4, so the actual lights being transmitted are not the same though they both have the same f-stop.
Originally by user2476. Source · Licensed CC BY-SA 4.0
user2476
14y ago
0
Generated from our catalog & community — verify before relying on it.
An f-stop is a geometric value: focal length divided by the diameter of the entrance pupil. It tells you the lens’s theoretical light-gathering ability.
A T-stop (“transmission stop”) adjusts that number for real-world light loss inside the lens. As light passes through glass elements, a small amount is absorbed or reflected, and lenses with more elements can lose more light. So the T-stop indicates how much light actually reaches the sensor or film.
Because of those losses, a lens’s T-number at a given aperture is always the same as or higher than its f-number. For example, a lens set to f/2.8 might transmit light more like T/3.2.
This matters most in cinematography and video, where exposure must match closely across shots and lens changes. Small exposure differences that might go unnoticed in single still photos can be obvious in moving footage. T-stops let filmmakers set exposure based on actual transmitted light, not just theoretical aperture.
In short:
- f-stop = theoretical aperture value
- T-stop = actual transmitted-light value
- T-stops are especially useful when exposure consistency is critical
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