If you spend your whole life shooting photo and never trying video, you might never even hear of a T-stop. However, they can tell you some very helpful information about your lenses.
Coming to you from ZY Productions, this great video talks about the differences between f-stops and T-stops. Whereas an f-stop is the ratio of the focal length to the diameter of the entrance pupil, the T-stop is the f-stop divided by the square root of the lens' transmittance. In other words, you can think of the T-stop as the f-stop adjusted to account for light losses as light passes through the lens elements on its way to the sensor. Note that the T-stop is only concerned with the effective f-stop as related to light transmittance; for depth of field purposes, the normal f-stop is still the correct measure. In addition to what the video mentions, T-stops are particularly useful to filmmakers because they ensure consistency in exposure across multiple scenes, lighting setups, lenses, angles, etc. While such inconsistencies could be corrected in post, it saves a lot of time and production money to get them right in camera. Check out the video above for more on the topic.