>>> If you didn't mind extending your test, what happens at around 35 >>> meters, which would be within the focal range of the A*300 (with >>> depth of field effect) but at infinity for the F*300? >> >> Chet, I'm glad you were asking Dave that question, because I >> don't understand the question - <g>. (Sorry.) >> >> Fred
> Oh well. Mine is a bad question then! But if I restated it . . . Well, Chet, it may or may not be a "bad question", but there can be confusion in the mind of the reader even for good questions - <g>. I will try to answer your question, but I am not clear exactly what you might be asking, and I also am far from knowing very much in the way of optics, so this reply may well fall flat on its face... > It seems to me that for 300mm lenses the more automatic the lens, > the less the bokeh. (I don't know!!!!) I don't think that how "automatic the lens" is (whether you are referring to autofocus or to automatic stopdown, or?) has ~anything~ to do with bokeh (or with DOF). > It goes back to something I was wondering about earlier when I > noticed that the SMC 300 is marked to focus to 300 feet before > infinity; the M* and A* dropped to 150 feet before infinity; and > the F* has dropped to 60 (or 80?) feet before infinity. Those numerical marks (if you are referring to the distance scales on the focus rings) don't have anything to do with either bokeh or DOF. The SMC K 300/4 can focus to 300 feet before infinity, to 150 feet before infinity, and to 60 feet before infinity. The M*/A* 300/4 can focus to 300 feet, to 150 feet, and to 60 feet before infinity. And (guess what), the F* can do the same... The distance between the numbers, or "how fast" the numbers change on different lenses, has to do with how fast the helicoid mechanism in a lens causes the focus to shift when putting that mechanism into motion, whether by manually turning a focus ring or by having an autofocus motor drive the mechanism. > So what happens if the subject is at 200 feet (with a distant > background)? Would the SMC 300 display DOF effects, whereas the > F*300 would have it all at infinity????? The main effects on DOF (of a lens focused near infinity) are the focal length and the aperture, I would think. So, an F* 300/4.5, an A* 300/4, and a K 300/4 would all show about the same DOF when focused at the same distance, if their apertures were all set to the same value (f/5.6, say). And this would have nothing to do with the spacing of the distance numbers painted on the lens barrel or the aperture ring. Wide open, I would expect the F* 300/4.5 to have a ~slightly~ more generous DOF, simply because it has a slightly smaller maximum aperture, but otherwise I would expect all of these 300mm lenses to have approximately the same values for DOF, aperture for aperture, and distance for distance. The big effect of distance on DOF is to shorten the DOF as the FL gets shorter. However, three different 300mm lenses,, all focused on a plane, say, 25 feet away, would all have the same DOF, if they are also set to the same aperture, I believe. I may be wrong here (not being anything but an ~armchair~ optical engineer - <g>), and I may still be missing the point of your question entirely, but I just thought I'd take a stab at your question. (Can someone else jump in here and shed a little light on this?) Fred
