From: "Steve Hodges" <shodges@iinet.net.au> > 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. Perhaps, but while you are technically correct, no one I know (except lens designers) refers to entrance pupil when discussing day to day photography, though I'm sure that somewhere there are some. For instance, you referred to it and have stimulated an excellent discussion. Frankly, I've only referred to "pupil" when discussing telescopes, spotting scopes and rifle scopes and then it was "exit pupil". The point is we all say, "aperture." We (in practice) use the term to define either the method of controlling the intensity of light at the film plane, or the DOF. I suppose we could all run around using the term "effective aperture" instead of just "aperture", but you and I know we're not going to do that - at least most of us aren't. We (and all the magazines) are going to continue to use "aperture" even if "effective aperture" or "entrance pupil / focal length" defined these properties. Even this is not correct because all lenses both absorb and reflect some of the light impinging on them. This means that effective aperture for DOF purposes is not the same as effective aperture for intensity control purposes and may vary from lens to lens. If control of light intensity is our main purpose, I suppose it could be argued that we should adapt the "T(stop)" system used by motion picture photographers, but we are not going to do that. We are all going to continue to use the term "aperture" and it still actually refers to the mathematics of a pinhole. When the camera manufacturer says you have just purchased a lens marked as having an aperture of 1:1.2, they are still referring to effective aperture rooted back to pinholes. > I'll simply overlook the other silly mistake in defining it as "aperture > over the focal length". 1:2.8 (ratio emphasized) = 1 divided by 2.8 = 0.357 = aperture over the focal length. It's the square of this number that gives the ratio of light intensity impinging on the film relative to 1 or diameter = focal length. An aperture of 25 mm with a 50 mm focal length is 25 / 50 = 1/2 = 1/4 the light intensity you would have if the aperture was 50mm. This would be an f/2 lens and may be marked 1:2 or (rarely) 1/2. > 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). Of course you are correct. I was referring to the diameter circle. I guess I thought that here on this list that would be understood. My error. I know you think me ignorant, else you would not have made that statement. I do have degrees in mathematics and engineering, have been active in photography as a hobby for 45 years and operated my own darkroom for a number of years. (I leave it to the technicians now.) My job requires me to do some occasional technical photography. If you wish to have a discussion regarding such things, I assure you that though I'm no expert, I am at least acquainted with the basics regarding aperture and can trace lens design back to Maxwell's equations. > > F 0.5 or f 0.5 is some kind of nonsense unless you're trying to make > > up some convention of your own. Sorry for my unwarranted remark re: your ass. :)
