Bob Blakely wrote: > > Further, 1:2 and 1/2 are ALWAYS unitless ratios expressing aperture over the > focal length, whereas f/2 is ALWAYS an actual aperture size measured in > linear units (a = f/2). If the focal length (f) is 50 mm, then f/2 means the > aperture is 25 mm. If the marking is 1/2 or 1:2, then this is ALWAYS the > ratio of the aperture to the focal length. Actually that is incorrect. You should refer to the entrance pupil when you say that. 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'll simply overlook the other silly mistake in defining it as "aperture over the focal length". 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). > F 0.5 or f 0.5 is some kind of nonsense unless you're trying to make > up some convention of your own. Oddly enough, that was exactly my argument. Love and kisses (your ignorant ass friend on photoforum) Steve p.s. Jacobson, Ray & Attridge, The Manual of Photography (8th ed), pp 48-49 p.p.s. No need to apologise :-D (You're not normally this abrasive -- last time we disagreed you were quite the gentleman)
