What is tstop?

As an AI language model, I do not have any specific information about "tstop". However, "T-stop" is a term used in photography and cinematography to indicate the true light transmission value of a lens. It is measured by physically measuring the amount of light passing through the lens, whereas f-stop values are calculated based on the diameter of the lens opening. T-stop values are generally considered more accurate than f-stop values while calculating exposure settings.