Archived posting to the Leica Users Group, 2005/08/31
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Thanks Marc > From: Marc James Small <msmall@aya.yale.edu> > Reply-To: Leica Users Group <lug@leica-users.org> > Date: Tue, 30 Aug 2005 19:08:35 -0400 > To: Leica Users Group <lug@leica-users.org> > Subject: Re: [Leica] Hyperfocal Focusing > > At 07:25 PM 8/30/05 +0200, Philippe Orlent wrote: >> How do you come to a CoC of 1/1000, thus 0,001? >> >> My very handy hyperfocal chart takes 0,03, thus 1/33, as CoC, making a >> difference per type of camera >> (see http://www.vividlight.com/articles/3513b.htm and >> http://www.vividlight.com/pdf/hyperfocal.PDF) >> >> What determines these numbers? > > Let's get back to basics here. The Gevaert Manual of Photography (5th Rev > Ed, 1962) carefully reminds us on p. 31 that "the conception of depth of > field is purely relative and depends on the criterion of sharpness > considered necessary for any particular application". That means, DOF > varies substantially depending on the actual sharpness necessary for the > job at hand. > > This depends in large measure also on the quality of the film used. The > "circle of confusion" is a mathematical statement of the resolution > required. A rather grainy film will permit a low circle of confusion; > that is, if the film grain comes to, say, 1mm, in diameter, than the circle > of confusion required will be 1mm (roughly 0.04" or, in franctions, 4/100") > That would be a really coarse-grain film. More common are grain sizes in > the area of 0.1mm, or 1/250", which would be that of, say, a relatively > high-speed older film such as Royal-X or the like. In today's world, > finer-grained films with higher speeds are readily available, so that it is > probably the case (I have NOT looked any of this up, but I am relatively > certain of the data presented), a rough grid could be developed as follows: > > slow films (say, 100 Delta) 1/1000" or 0.025mm > medium-speed films (say, PanF) 1/750"or 0.034mm > fast films (400 Delta &c) 1/500"or 0.05mm > > This is supposed to be the size of the average grain actually produced by > these emulsions and, agian, I've not checked this out against Ilford's data > but I am fairly certain that I am in the ball-park. > > The basic formula is: the hyperfocal distance in Imperial measures equals > the focal length of the lens in inches squared divided by 12 times the > aperture, this quantity then being multipled by the reciprocal of the > circle of confusion in fractions of an inch. > > So, again, let us consider this 35mm f/3.5 lens when shot at f/3.5 under > varying conditions of film resolution. > > With a slow film, we get the following: > > The lens focal length of 35mm in inches is 1.378". This quantity squared > is 1.9. Divide this by 12 x 3.5, or 420, to achieve a factor of 0.0045. > Multiply this by the reciprocal of the circle of confusion your film > requires, and you get the following: > > 0.0045 times 1/1000 (low-speed film) = 4.5 feet > 0.0045 times 1/750 (medium-speed film) = 3.4 feet > 0.0045 times 1/500 (high-speed film) = 2.26 feet > > I believe that my maths are right but, then, maybe not. In any event, the > results I get are as follows: > > 100 ASA film: set the lens to 4.5 feet and everything between 2 feet 3 > inches and infinity will be focus to the limit of the film. > 200 ASA film: set the lens to 3.4 feet and everything between 1 foot 8.4 > inches and infinity will be in focus to the limit of the film. > 400 ASA film: set the lens to 2.26 feet and everything between 1 foot 1?" > will be in focus to the limit of the film. > > ,Note that the range of the focus increases as the circle of confusion is > reduced. > > Trust me on this one: the press photographers who dominated American > photo-journalism from the 1920's to the 1960's could run this formula in > their heads while they were humping their Speed Graphics from one location > to another. These guys really lived up to the Ben Hecht iconography and > most smoked cigars and most carried hip flasks and all of that -- but they > could crank out a hyperforcal distance in a heartbeat. It was the stuff of > their lives, as they only got ONE chance to get a GOOD shot of, say, the > local Mayor being hauled to the jailhouse or of a local pastor fleeing a > whorehouse. So, yes, hyperfocal distances were a key part of their lives. > (I had an encounter with one of the Speed Graphic guys from the dead and > much-lamented Baltimore News-American around 1975 at nine in the AM: he > was already drunk but explained hyperfocal distance to me so clearly that > the concept has lived with me to these days.) > > Bear in mind that the circle of confusion varies from with the type of film > used. That is why those Depth-of-Field scales printed on the lenses are > most suspect, unless you know just what circle of confusion was used in the > calculations to produce these ranges. In some cases, such as the 4.5/21 > Carl Zeiss Biogon, it really doesn't matter due to the extreme shortness of > the lens coupled with the slow basic speed of the lens, but it does make a > lot of difference with, say, a modern 1.4/35 Summilux-M whether you are > using your reserve stock of PanF or 3200 Delta. > > 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 >