Re: [APML] Film Thickness and Focusing

[Jerry Lodriguss <jerry@astropix.com>]

Hi Thom,

The depth of focus for an f/6 system is, I believe, about 0.005" ballpark.

Chuck uses some high quality Scotch tape on his pressure plate to make up 
the difference when he shoots Techpan.  You would probably need two 
different camera backs if you plan to shoot E200 and Techpan and use this 
method.

Note that the majority of the film's emulsion will be the film's thickness 
in front of the pressure plate, NOT on the inner film rails of the camera. 
(I say majority because unless you are using a vacuum back, the film will 
not be held perfectly flat.)

IF you could accurately focus to exactly 0.005 inches in front of the 
pressure plate, then the depth of focus would cover you, I think.

The best way to do this would be to put a knife edge 0.005 inches below the 
outer rails.

Jerry


At 08:19 PM 5/20/2003 -0400, you wrote:
>Hey all, getting tired of the "by guess, by golly" focusing method but
>lacking the funds for a real focus aid I have been researching the
>various homemade knife edge and ronchi screen focusers.
>
>My question is what is the depth of focus(field?) for an 8" f6
>combination?  I ask because today I took some film to work and measured
>the thickness of the leaders using some precision gauging in our quality
>lab. My results were:
>Elite Chrome 100                .0061
>Elite Chrome 200                .0062
>Old (good) Supra                .0060
>Tech Pan                        .0040
>
>Do I need to create a set of shims for the difference between Elite
>Chrome 200 and Tech Pan?  These are my two primary films.
>
>As always, thanks in advance.
>
>Thom Iwancio
>Elgin, SC
>www.machunter.org
>
>
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