What is F-stop? What is T-stop? What are the differences between F-stop vs T-stop? This is the subject of our article today. As you know, there are some values on the lenses. One of these values diaphragm value is. This value, which determines the amount of light that will fall into the sensor, varies according to the structure and quality of the lens. This factor, which also affects depth of field, has been expressed with the F value in digital and analog photography for the last 10 years. While photographers have been telling about the aperture value with the terms such as F 2.8 – F5.6, we have started to see that this value is also expressed with the T value with the cheapness of cinema, lenses and cameras in recent years. So what is F Stop and T Stop? What are the differences between them?
What is F-Stop?
F-Stop is a value created using purely mathematical calculations, which usually express the aperture we see in photo lenses. Normally, light enters the lens, passes through the diaphragm leaves and lenses and breaks, gathering at some point behind the lens, then the broken lights fall on the sensor and form the photograph. In this process ranking, the measurement of the F value is actually the first. The F value is a value created based on the light value entering the lens. On the other hand, f stop numbers are found mathematically on the basis of dividing the focal length of the lens by the diameter of the diaphragm. For example, if the aperture diameter (lens diameter of the lens) of a 50mm lens is 25mm, the lightest aperture value of that lens is calculated as F 2. So the value produced is purely theoretical.
Since F is a theoretical calculation, the losses of light entering the lens and passing through the lenses are not taken into account, which creates a difference between two lenses containing a different number of lenses but having the same aperture value. For example; At 24mm f / 2.8, a slightly brighter image is obtained, which is noticeable to the eye, while it creates a brighter image than 28mm due to the use of more lenses at 100mm f2.8. This small difference does not make much difference in the field of photography. You can tolerate this little difference by playing with ISO and shutter speed, but this is not the case in filming. Generally, cinematographers set an aperture value and all shots of that scene are performed with the same aperture value.
For example; When the Director of Photography draws the player at a shutter value of 50 at 28mm f / 2.8 and then pulls the same player at 1 shutter at 100mm f / 2.8 at the back of the plan, there is a slight difference, and this is very unwanted. For this reason, the diaphragm is expressed with a different measurement called T-stop in lenses used in cinema, different from F-stop.
What is T-Stop?
T-stop is a value that expresses the diaphragm value, which is generally used in cinema lenses, and also takes into account the losses of light in the lens. This value, unlike f stop, creates the diaphragm value by taking into account the refractions of light within the lens. For example, 85mm t1.5 and 24mm t1.5 give exactly the same light value. For this reason, all lenses in cine serial lens sets consist of lenses with the same T value.
Since continuity is a very important factor in filming, lenses with T value are used in order to avoid exposure difference between lens changes. In the photo, it is not important to use T or F value since there is no continuity in lens changes.
Is F-Stop or Better T-Stop? Which one should I buy the lens?
First of all, it should be noted that whether a lens is T-stop or F-stop is not a value that indicates the sharpness, color deviation value and quality of that lens. In other words, it can not be interpreted that lenses with T-stop or F-stop are better, but using lenses with T-stop value in filming is considered more ideal in the ideal world. For this reason, first decide what you want to shoot while buying the lens, and then look at the aperture, focal length and quality of that lens and see if it uses T-stop or F-stop. If you are going to use the lens only for filming, choose the one with the T value, but if you say I will take both photos and videos, the lenses with the F value are the most suitable for you.