Archived posting to the Leica Users Group, 2005/08/29
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Marc, Somewhere I think you mixed up cm and mm? Perhaps circles of confusion 1/1000 is 0.0025mm or a factor of 400? Chris S. At 06:47 PM 8/28/2005, you wrote: >You can do this in one of two ways. > >First, you can use the rather crude depth-of-field scales on the lens >itself; these are approximations but are, in general, accurate. > >Second, you can do what real photographers do: use the damned formula. >This is a matter of the Leica Mantra set out in every single volume of the >LEICA MANUAL from Edition 1 to Edition 15. It is available in all >editions of the Gevaert Manual. Und so Weiter. The old press >photographers knew this one by heart: > >hyperfocal distance = focal length of the lens squared divided by twelve >times the f/stop, this in turn being multiplied by the reciprocal of the >circle of confusion. > >The "circle of confusion" is the smallest resolution of which the film is >capable; this is avariable, of course, but, for most purposes, this has >been standardized for the past fifty years at 1/1000" (roughly, 0.025mm). >Allow me to give an example, based on that 3.5/3.5cm lens which led to this >discussion. > >Let us suppose that the lens is set to f/3.5, or wide open. > >The formula produces 1225 (35mm squared) divided by 42 (12 times f/3.5) or >29 1/6. Now, we multiply this by 1/0.025 -- note that I am sticking to >metric units, though the same could certainly be handled in Degrees Rankine >and the like in Imperial Units.) -- or a factor of 40, to produce a final >result of 1166 2/3mm for a final hyperfocal distance of 11.66 meters or 38 >feet, 3 inches. That is, setting the lens at f/3.5 and at 38 feet distance >on the focusing scale will insure that all objects between half of this, or >from 19 feet, to infinity will produce a sharp image on film capable of >producing an image resolution of 0.025mm; most modern films can exceed >this greatly, I suspect. > >Note the differences between Imperial units and Metric units and be certain >to convert from MM to CM to Meters. (There is as much range of confusion >in the one system as in the other, and the combination of the two is an >invitation to disaster, as witness JPL's loss of a Mars probe some years >back, though WHY any scientific agency has not properly defined its >standard system of measurement escapes me, and oy vey!) > >That is the heart of hyperfocal distance caluculations though there are a >number of quick formulae used over the years. Ed Meyers and Bob Shell, >neither of this list, probably would be able to set out the system more >elegantly. > >Marc > >msmall@aya.yale.edu >Cha robh b?s fir gun ghr?s fir! > >NEW FAX NUMBER: +540-343-8505 > > > > > >_______________________________________________ >Leica Users Group. >See http://leica-users.org/mailman/listinfo/lug for more information Chris Saganich, Sr. Physicist Weill Medical College of Cornell University New York Presbyterian Hospital Ph. 212.746.6964 Fax. 212.746.4800 A0049