On this video, Gerald Undone looks at the important differences between f-stops and T-stops, what each one is used for and why and their specific formulas.
f-numbers are a ratio between the focal length of the lens and the diameter of its entrance pupil. f-numbers are a good measurement for the size of the focal plane (and hence background blur at different distances).
f-numbers are not the most accurate measurements of the exact amount of light that gets through the lens. If you are shooting a movie and you are changing a lens you don’t want your exposure to be different (which might very well happen if you will only be using f-numbers). T-stops, on the other hand, will give you a good measurement of the light transmittance of a lens – or exactly how much light passes from the front to the back of the lens – the closer to 100 the better the transmittance and the closer you get to the correlative f-number (typically you might find t-2.9 lenses, for example, having f/2.8 ratings – however the opposite isn’t necessarily the case – if you have an f/2.8 lens you can’t just assume it will have a t-2.9 or t-3)
At the end of the day a t-stop is more complex to measure and hence lenses with an official t-stop will typically be more expensive and they will also be typically used when consistent light transmittance (or exposure consistency) is important – light in higher budget video productions and movies.
On LensVid we have a very extensive section devoted to sound recording for video productions. You can find many of Niccolls/Drake videos here on LensVid. It is also worth checking out some of Gerald Undone’s other videos and we will be adding them to this sub-category on LensVid.
Iddo Genuth 
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