T-Stops are exactly the same as F-Stops and are used in the same way – to control the amount of light that reaches the unexposed film stock. The difference between the two is that a T-Stop has been calculated for an individual lens, where as F-Stops are calculated based on a fixed formula which does not take into account variations found in individual lenses.
When a lens is manufactured, there will always be slight variations in the focal length and aperture sizes. In most cases, these variations will have no noticeable effect on the focus/exposure of the film, but in some extremely delicate cases, it will. In order to provide cinematographers, with the most accurate information possible, manufacturers calculate the stops of a lens based on its individual behaviour. The results of these tests are then included on the lens as T-Stops (i.e. more accurate F-Stops) and one of several reasons why pro lenses are incredibly expensive.
