Archived posting to the Leica Users Group, 2005/08/28
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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