You have probably heard both F-stop and T-stop values being used when referring to aperture. Both terms are often confused with each other but their meanings are slightly different.
F-stop refers to the numerical representation of the size of the lens aperture in relation to the focal length. This means an f-stop value will tell you how much light your lens is letting in.Â A lower F-stop number (eg.1.2, 1.4, 1.8) will let more light in than a higher f-stop (eg.8, 11, 16, 22).
When you move down one F-stop number, for example from 2 to 1.8, you double the amount of incoming light. Stepping down two F-stop numbers will times the amount of incoming light by four and so on. The same will happen when increasing the F-stop number, with the amount of incoming light halving with every step.
Moving between F-stop values is referred to as “Stopping Up” (if the number is being decreased), or “Stopping Down” (if the number is being increased). Every lens will give the user the ability to step between most of the standard F-stop increments from 1-22. However, sometimes photographers and filmmakers want a very specific F-stop number which may be half, third, or even quarter steps between F-stops. Many lenses give the user the ability to do this with some getting down to decimal places between standard steps.
Although F-stops are an accurate representation for the amount of light allowed into a lens, they are not always an accurate measurement of the light which reaches the sensor. This is because small amounts of light get lost, mainly through reflecting off individual lenses before reaching the sensor. However, some high end lenses reduce reflections and therefore increase the percentage of entered light reaching the sensor. Most lenses allow around 60% -90% of light to pass through, with some more expensive lenses allowing a little more.
F-stops are therefore not an accurate indication of the exact amount of light passing through a lens, just the amount of light allowed to enter. A T-stop value refers to the actual amount of light passing through a sensor. This takes into account the percentage of light that comes through a lens and mixes it with the f-stop number. For example, a 100mm lens at f/3 with a light transmittance of 75% will have a t-stop of 4. Similarly to F-stops, the higher the t-stop value, the darker the image. T-stop value is worked out using the following equation.
T-Stop = F-Stop
Lens Transmittance %
T-stops are simply a better indication of the exact amount of light actually entering the sensor. Since most lenses don’t allow much light to reflect anyway, T-stop values aren’t usually taken much notice of in the photography world. However, they are generally usually more important in the filmmaking industry and can be a very important value to many film makers.