Archived posting to the Leica Users Group, 1999/08/31
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Jim, I agree that the point of my post may have drifted from that of the original poster. What caught my attention was specifically that "rule" you quoted (which I have heard repeated often as gospel truth and only today questioned). I'm still baffled as to how the 1/3 - 2/3 rule can be supported scientifically, when it apparently isn't supported empirically. My assumption is that when Leica engraves DOF markings on their lenses, the two markings (near and far) indicate the same size CoC, for the same degree of "relative unsharpness" of a point source. So, take my experiment of focussing the 35/2 at 2m and f/16. The lens DoF scale (engraved on there by gnome #134) clearly indicates that points at 1.1m and points at 10m will have the same CoC, with points at 2m being sharp. The distribution in front/behind the point of focus is a long way from 1/2 - it's about 1/9. So how can this "rule" be supported? The f-stop used does alter this ratio: at f/16 focussed at 2m the distribution is 1/9, at f/11 it is 1/3.5, at f/8 is 1/2.5, and at f/5.6 and f/4 it is right about 1/2... So what gives - is it Leica's technique for calculating "acceptable CoC" at different apertures, or is the rule an approximation for wider apertures only? Or is the subject actually more complex than that? No points for suspecting that the prize is behind door number 3... Paul Chefurka