All the lenses available in the market have an adjustable aperture or iris which controls the amount of light that passes and strikes the film. A wide opened aperture allows more light to pass through to film than a small opening, it differs from the available source of light present to film the scenario. The numbers which defines these openings are called an f-stop. It’s a mathematical calculation equal to the focal length of the lens divided by the diameter of the aperture opening. The standard series of f-stop numbers are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 ans so on.
All lenses absorbs some amount of light passing through them, therefore there is a precise representation of the amount of light reaching the film through the lens. This calculated number is called a t-stop. A t-stop is the actual amount of light that transmits through the lens at a particular aperture openings. The t-stop is more accurate and should always be used while setting up an exposure meter.
The t-stop numbers are the same as f-stop numbers, but a t-stop is not the same as an f-stop. Many times a lens will be calibrated for both f-stops and t-stops. When setting the exposure precisely on the lens, you should always use t-stops. When measuring the intensity of the light with a light meter or when calculating depth of field, you should always use f-stops. Whenever you film at a frame rate other than 24 fps, you must change the stop compensate for the new frame rate.