T-stop
From The Glossary of Digital Photography
Refers to transmitted light. It is also called a T/stop. Related to f-stop, the T-stop does not give the same result. The T-stop is dependent upon the light passing ability of the glass as well as the optical system. The T-stop aperture indicates the actual amount of light going through the lens's aperture. F-stops show the size of the aperture opening compared to the focal length of the lens. Many lenses with T-stops mark the T-stops in red, with the f-stops marked in white on the same lens barrel. Since zoom lenses often have light loss in different parts of the zoom range, using T-stops on them will help get the best exposure. Typically, T-stops are used on motion picture camera lenses because they want more precision in their settings. See also f-stop
