T stop

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T stops are similar to f stops and are used to calibrate the iris on a lens.

They are a measure of the actual amount of light which is allowed to pass through the lens at a specific T stop setting on the iris. T stops are more accurate than f stops because they are calibrated for the individual lens by using a light meter to determine how much light passes through the lens at each iris setting. This takes into account any light losses which may occur as the light passes through the lens. F stops are calculated mathematically for any lens and do not allow for any such light loss.

In practice, T stops and F stops are very close to each other and many cinematographers treat them as the same thing.

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