What’s the practical difference between f-stops and T-stops in video, and why can movie sets look bright at seemingly small apertures?

Question

I’m a stills photographer and was talking with a focus puller on a film set. We hit a terminology barrier when discussing exposure: I was thinking in f-stops, while they were talking in T-stops.

On a large sound stage, the image looked well exposed even though the depth of field seemed to suggest a relatively small aperture (roughly like f/6 to f/8 in stills terms). That made me wonder:

- How do T-stops translate to f-stops in practice?
- Can a lens somehow transmit lots of light while still giving the depth of field of a small aperture?
- Or is the explanation more likely things like higher ISO, longer shutter time in cinema, or simply very bright lighting?

I later saw that one lens set included primes roughly in the 24mm T2.2 to 135mm T2.4 range.

user37674 · Answer

There's nothing here to grasp. T-stop is the same thing as f-stop, except the manufacturer or a third party has physically tested the lens to find how much light it transmits, rather than making a theoretical calculation based on the size of the exit pupil, which is where the f-stop measurements come from.

When a manufacturer specifies T-stops on their lens, they are making a guarantee that the lens has been physically measured and confirmed to allow the specified amount of light through to the back of the lens. The real measurement of light transmission will always be lower than the theoretical one, because some light is absorbed by the body of the lens, or otherwise lost during its journey. Consequently, at a given f-stop on a lens, the T-stop will be higher. This affects exposure, but not depth of field.

In videography, it's more important to make sure exposure is consistent than it is in photography, because editing film after the fact is time-consuming and expensive. Different shots from the same scene may be taken on different days and under different lighting conditions, and even with different lenses. Measuring the real T-stop value of lenses allows you to interchange them without unexpected under- or overexposure.

If your question was "How are you getting so much light at such an apparently small aperture?" and the response was "..but we don't use f-stops, we use t-stops.", then what you received was a non-sequitur. They use the same exposure triangle as photographers, and so would have to be using a combination of slow shutter speed and high ISO. As pointed out by some other answers/comments, videography can get away with slower shutter speeds and higher ISOs than photography.

Depth of field is determined entirely by f-stop and magnification of the subject at the focal plane (on the sensor). To increase depth of field, you either have to:

Decrease magnification - i.e. use a smaller sensor with pixels more densely packed or reduce the size of the subject in the frame, which is not a compromise that can be made while filming or
Decrease the size of the exit pupil (increase the f-stop).

If a good depth of field is critical, then you want a well-designed lens where the T-stop is not too much higher than the f-stop. If it is a lot higher, then you have to compensate by decreasing the f-stop and the depth of field with it.

What’s the practical difference between f-stops and T-stops in video, and why can movie sets look bright at seemingly small apertures?

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