What’s the Difference Between T Stop VS F Stop?

Aspiring cinematographers frequently focus on understanding the F-stop but sometimes overlook the importance of the T-Stop. The difference in T Stop VS F Stop is something that many cinematographers don’t fully focus on. Many focus solely on the t-stop whereas many photographers will focus on the f-stop, but what’s the difference between the T stop VS the F stop?

Understanding the T-Stop

The T-stop is the common measurement that is used on cameras and camcorders that are produced for the purpose of film or video production. Similar to the measurement of the f-stop, the t-stop represents the f-stop correction based on the actual ratio of light transmittance. 

When you look at the difference between a t stop VS f stop, the primary adjustment is the calculation.

The t-stop is calculated by dividing the f-stop by the square root of light transmittance such that a lens with an aperture of 2.8 that features a 90% light transmittance will have a t-stop value of 2.8 divided by the square root of .90 or a value of 2.953 which would then be rounded up to a t-stop value of T/3.0.

Understanding the F-Stop

In photography, the F-stop is one of the first measurements that you’ll learn about. These measurements of the lens are incredibly important for you to understand as they apply to exposure. 

When describing an F-stop, the photographer or cinematographer is referring to a ratio of the total focal length as it is compared to the overall diameter of the entrance pupil of the lens. F-stops are calculated by taking the focal length and dividing that number by the diameter of the entrance pupil. 

The f-stop is the amount of light that is transmitted through the lens. So, a 50mm lens that has an entrance pupil diameter of 25mm would result in an f-stop of 2. You’ve probably seen this type of lens before, referenced as an f/2 lens.

While the f-stop plays a key role in the amount of light transmitted through the lens, various other factors can also come into play including individual lens elements, whether or not there is a high level of reflection in the lens barrel, and the overall quality of the glass that the lens is constructed from.

What is the Purpose of T-Stops & F-Stops?

Filmmaking requires an understanding of t-stops vs f-stops in order for the cinematographer to grasp an accurate value of light transmittance through the lens.

While it’s expensive to accurately measure t-stops compared to the lesser requirements in measuring an f-stop, we tend to see the t-stop used in filmmaking more so and the f-stop used in photography.

The true difference between t-stop vs f-stop is in the calculations, but between the two the overall data that is provided after the calculations is similar in terms of that both are providing the cinematographer or the photographer with insight into the values of a lens.

T-stop measures light transmission and f-stops measures focal length. F-stops are a theoretical value and t-stops are an actual tested value meaning that the t-stop is more accurate.