Newsgroups: comp.sys.sgi
Path: utzoo!utgpu!watserv1!watcgl!imax!dave
From: dave@imax.com (Dave Martindale)
Subject: Re: photographing the screen
Message-ID: <1990Oct30.173541.14407@imax.com>
Organization: Imax Systems Corporation, Oakville Canada
References: <26308.27257d14@kuhub.cc.ukans.edu> <1990Oct25.183533.19456@odin.corp.sgi.com>
Date: Tue, 30 Oct 90 17:35:41 GMT

Some additional tips on exposure:

1. If you have a spotmeter available, this is the most accurate way I know
   to set exposure:  Plot a square of full-intensity white on the screen,
   and read it with the spotmeter.  Then give the film 2.5 stops more exposure
   than the spotmeter reads.

2. If you don't have a spotmeter, but do have a through-the-lens meter on
   your camera, fill the screen with white and take a reading, then give
   2.5 stops more exposure.

3. Read the data sheet supplied with your film, and look for "reciprocity
   corrections".  The manufacturer will often recommend additional exposure
   when making longer exposures in the 1 second range.  However, shooting
   a screen does not really give 1 second of relatively dim light - it
   is 60 successive exposures of somewhat brighter light - so you may need
   less correction than the data sheet says.

I suggest using full-intensity white instead of mid-grey, since few people
will actually know how to display a true mid-grey on their CRT.

Where does the magic "2.5 stops" come from?  There are two ways of thinking
of this:

 - the light meter reads the white area as if it were mid-grey (about 18%
   reflectance).  The difference between 18% and 100% is about 2.5 stops,
   so opening the iris by 2.5 stops gives the correct exposure.

 - taking an exposure meter reading of a particular tone in an image,
   and then giving 2.5 stops more than the meter indicates, will place
   that tone up near the "shoulder" of the characteristic curve of
   a slide film - thus causing it to reproduce as fairly bright white,
   while still retaining detail in the whites.