X-Authentication-Warning: delorie.com: mail set sender to geda-user-bounces using -f
Date: Thu, 3 Apr 2014 11:02:11 -0400
Message-Id: <201404031502.s33F2BUe008492@envy.delorie.com>
From: DJ Delorie <dj AT delorie DOT com>
To: geda-user AT delorie DOT com
In-reply-to: <20140403100402.53feb6b6@richard-laptop> (message from Richard
	Bown on Thu, 3 Apr 2014 10:04:02 +0100)
Subject: Re: [geda-user] printing from PCB
References: <20140402112140 DOT 5df5a754 AT richard-laptop>
	<201404021726 DOT s32HQYBe009236 AT envy DOT delorie DOT com> <20140403100402 DOT 53feb6b6 AT richard-laptop>
Reply-To: geda-user AT delorie DOT com
Errors-To: nobody AT delorie DOT com
X-Mailing-List: geda-user AT delorie DOT com
X-Unsubscribes-To: listserv AT delorie DOT com
Precedence: bulk


> 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.