Archived posting to the Leica Users Group, 1999/07/28
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]<<<<What does the 'f' in f-stop stand for? (I've asked people and looked in books, and had to admit to the person who asked me - haven't got a clue!)>>>> "f" in "f-stop" means "factor." For example, a 50mm focal length lens with a diameter of 25mm (optically) is an F:2 lens, or a factor of 2 of the focal length. When you close down the lens diaphragm blades so that the opening is 12.5mm diameter, you have F:4, or a factor of 4 of the focal length. The same factor among different focal lengths gives the same brightness, more or less -- different lenses with differing number of elements, mirror size in mirror lenses, etc., may vary somewhat in the amount of light actually transmitted to the film. To add a little more, there is something called "T-stops" that are actually measured amounts of light transmitted, so a given T stop between two lenses should give exactly identical exposure; but, I have only heard of T-stops being used in the motion picture industry. Tom Schofield