[Leica] Hyperfocal Focusing

[philippe.orlent at pandora.be (Philippe Orlent)]

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!
> 
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