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Date:	Thu, 3 Apr 2014 11:02:11 -0400
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Subject:	Re: [geda-user] printing from PCB
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> BTW what is a  Stouffer 21-step transmission wedge  ?

The first page of Google's results for "Stouffer 21-step transmission
wedge" gives lots of relevent information, starting with:

http://www.stouffer.net/TransPage.htm

A transmission wedge reduces the light hitting your film.  You do a
test exposure with a wedge in the way, and each "step" of the wedge
tells you if you have enough light to expose the film under it
*despite* that reduction.  The steps are calibrated so that you can
use the results and a little math to adjust your exposure time.

Most films want a "hold at 8" which means you need enough light/time
to just barely expose your film despite the reduction of light at the
8th step.  Due to the math of a 21-step wedge, this turns out to be 16
times the exposure needed to "just" expose the film without a wedge,
which is what most people eyeball it to.  Thus, most people
underexpose their films.

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