stop, lens stop, f-stop, t-sto

An adjustable iris Diaphragm that controls, in part, how much light reaches the film or video tube. Once you grasp the logarithmic progression, and the idea that the larger the number the smaller the opening (therefore less light) - it s a piece of cake. F-stops are the theoretical amount of light transmitted by the lens; t-stops, the actual amount. The difference is about 1/3 stop, often more with zooms.