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One of the first things you come across in photography is F-stop (from focal length “focal length”). It, as a measurement of the lens, is an important part of the exposure, like the T-stop (from the English transmission – “transmission”). We suggest you familiarize yourself with the article by professional photographer Gabor Holtzer in which he explains on the fingers the differences between T-stop and F-stop.

What does F-stop mean?

Each lens has an entrance pupil. The entrance pupil is that part of the lens where the diaphragm is located (and not the front of the lens). Its diameter is usually slightly less than the diameter of the front glass element.

F-stop is the ratio of the focal length to the diameter of the entrance pupil. So, if the focal length is 50 mm and the diameter is 25 mm, you have an f / 2 lens. If you close the diaphragm, the diameter of the pupil will narrow and less light can pass. The F-stop marked on your lens is the highest value.
However, the light transmittance is much larger than the f-stop ratio. There are many factors: glass quality, number of lenses, etc. They affect how much light reaches the sensor.

As a result, f-stop is proportional to the amount of transmitted light. This is not an absolute value.

What does T-Stop mean?

This is an f-stop adjusted for the actual light transmittance. That is, f-stop is the theoretical value, and t-stop is the actual value tested. You can calculate it by dividing the f-stop lens by the square root of the light transmittance.

So let’s assume that we have a lens with a maximum aperture of 2.8, capable of transmitting 90% of the light. Divide 2.8 by the root of 0.9. The resulting value is 2.953, which we can round to T / 3.0.

Thus, t-stop is a parameter that shows how much light the lens passes.
How can we determine the exact transmittance of light?

Unfortunately, there is no formula that would tell us the exact transmittance. If we want to know the exact amount of transmitted light, we must measure it. But this is not the thing that you can do at home – it is a meticulous process that requires a lot of input.

Some studios themselves measure the light transmittance, but usually it is not 100% accurate.

Which lenses have the highest transmittance?

The more elements there are in the lens, the lower the light transmittance. Each element of the lens slightly reduces it. So, for example, zoom lenses have a lower transmittance than lenses with a fixed focal length.

Much depends on the quality of the glass and the assembly. Cheaper lenses usually have lower quality glass. This means (among other things) that they let in less light than more expensive optics. Of course, there are exceptions. For example, Rokinon lenses with manual focus are not only of very high quality, but even have their analogues in movie lenses. Build quality inside the lens also matters. With better handling of internal reflections, less light will be lost.

What is the purpose of T-stops?

There are many reasons why you do not see the T-stop value in your everyday photo. One reason is that accurate measurement is quite expensive. Thus, companies do not even test their cheaper lenses for light transmittance.

Another thing is that in photography, f-stop parameters are more practical. Today, every digital camera has built-in metering and high ISO performance. Thus, you are unlikely to notice the difference between the T3.2 and T2.9 lenses (if both are F / 2.8), because your camera corrects this.

The area where T-stop values ​​are much more useful is video production.

Using T-stop to prevent exposure differences with movie lenses

When you always change angles, it is important to avoid differences in exposure between them. The easiest way to achieve this is to use lenses with the same T-stop.

On film lenses, t-stop is marked instead of f-stop. They also have the same body and weight. This feature makes it easy to balance and install additional equipment for filmmakers on them. In addition, they have removable focus and zoom rings. The optics inside these movie lenses are not much different from lenses for cameras. Look at the Canon 50mm f / 1.2 and 50mm t1.3 lenses – they have very similar optics in two different frames.

Why do most photo lenses use f-stop and movie lenses use t-stop?

There are three reasons why photo lens makers are not worried about t-stop:

measurement of illumination in the camera compensates for the slight difference in exposure between two different lenses with the same f-stop parameter, but different t-stop;
the biggest difference that you get is about 1/3 of the stop, which is corrected without problems with post-processing;

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