F-stop or f-number is the ratio of the focal length of a lens to the diameter of the entrance pupil. (Approximately the aperture diaphragm size in a symmetrical lens).
T-stop is a measure of the light transmission of the lens. It is related to f-stop by the efficiency of light transmission. A lens which transmitted 100% of the light entering it would have the same f-stop and T number.
To compensate for backlash in the mechanism, always set a lens diaphragm by moving from the widest opening to the desired aperture. This method takes up any backlash that may be present and provides the most accurate setting. (Reference: ANSI PH 22.90.)
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