Xref: utzoo comp.graphics:3708 rec.photo:3750
Path: utzoo!utgpu!watmath!onfcanim!dave
From: dave@onfcanim.UUCP (Dave Martindale)
Newsgroups: comp.graphics,rec.photo
Subject: Re: photographing computer screens
Message-ID: <16768@onfcanim.UUCP>
Date: 23 Nov 88 01:14:50 GMT
References: <18196@ames.arc.nasa.gov>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: National Film Board / Office national du film, Montreal
Lines: 38

In article <18196@ames.arc.nasa.gov> watson@ames.arc.nasa.gov (John S. Watson) writes:
> 
>Once everything is set up I run a program I wrote that take the three
>red, green and blue image files and make a grey scale "intensity" file.
>Then I display this intensity image.  Next, I turn out all the lights in
>the room and take a meter reading of the intensity image.

A much simpler method: fill the screen with full-intensity white.
Take a light meter reading.  Increase exposure by 2.5 stops.
This puts the brightest whites near the upper end of the linear portion
of the response curve for slide film.  With negative film, you might
give just a bit more exposure if you want more shadow detail.

(Note that 2**(-2.5) = 0.18, and thus that this method is also equivalent
to displaying and reading an 18% grey patch, except that by using white
you can get away with a less-sensitive light meter, and also don't have
to worry about how well your gamma correction has been done - i.e. whether
your 18% grey is really 18% of white).

>I want a long shutter speed to avoid flicker, 
>on the order of 10 seconds, so I set the aperture to 8 or more.

Most films recommend using colour filters to correct for colour shift
due to reciprocity failure with such long exposures.  An exposure time
of 1 second seems a good compromise - long enough that you don't see
a dark bar due to recording a non-integral number of video fields,
but short enough not to worry about reciprocity problems.
 
>Next I set my camera to expose the same frame 3 times, and set the 
>exposure to 1/3 of the meter reading I got from above (so for the 10 second
>example I'd expose with three 3.33 second exposures).

This is wrong - you should use the same exposure, not 1/3 of it.
Your light meter tells you to expose the film to the full-colour image
for, say, 1 second.  If you had a 24-bit frame buffer, you would do
exactly that.  Since you have just 8 bits, you do each colour separately,
but you still need 1 full second of red, 1 second of green, and 1 second
of blue to give the same exposure as 1 second of white.