> Actually that is incorrect. You should refer to the entrance pupil when > you say that. True ... > Because the relative aperture of a lens is calculated (for infinity > focus) as the focal length of the lens divided by the diameter of the > entrance pupil. > You would, naturally, be aware that the size of the entrance pupil and > the physical size of the aperture stop are frequently not the same. I knew someone else would step in and (correctly) complicate the issue. The physical diameter of the aperture in a 100mm f2.8 lens can be smaller (usually) or bigger than 100/2.8 mm. It depends on the "path of the light" internally (between the entrance and exit pupils). > Oh, and also the error in not identifying it as the diameter of an > equivalent circular aperture (or perhaps as the "mean diameter of the of > the actual aperture formed by the diphragm opening") rather than what > could be interpreted as some other measurement (radius or circumference > perhaps). Few modern diaphragms are circular: in the old old days they were ... when they were called stops ... > > F 0.5 or f 0.5 is some kind of nonsense unless you're trying to make > > up some convention of your own. If Micro$oft ever start selling digital cameras you can be damned sure they will invent some terminology just like that ... Bob "For every simple question there is a simple answer which is wrong"
