Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!samsung!aplcen!haven!rutgers!att!cbnewsj!jwi
From: jwi@cbnewsj.ATT.COM (Jim Winer @ AT&T, Middletown, NJ)
Newsgroups: comp.graphics
Subject: Re: photographing screens
Summary: Shutter speed
Message-ID: <2474@cbnewsj.ATT.COM>
Date: 28 Nov 89 15:12:01 GMT
References: <3777@hydra.gatech.EDU> <18678@watdragon.waterloo.edu>
Organization: AT&T Bell Laboratories
Lines: 65

> Robert Viduya writes:
> >Has anyone advice or pointers to information on how to photograph video
> >screens? 

rsingh1@dahlia.waterloo.edu writes:
> Shutter speed I used was, if I remember correctly, quite short.  I guess
> you want a shutter speed faster than 1/60th of a second (ntsc).  I played
> around with lots, so I might be wrong on this.

(sorry if I got the attributions backwards).

This information is WRONG! Your shutter must be open at least one full
(and preferably two full) screen write cycles which are usually 1/30 second. 
Speeds of 1/30 to 1/15 should work for most screens. (The shutter is a
traveling window. It is not all open at the same time -- the first
curtain opens and the second follows to close it resulting in a traveling
slit at high speed. To capture the screen, you must have the speed slow enough
that the entire shutter is open at once long enough for the screen to write
an image.)

To avoid loss of detail, it is essential to use a very solid tripod
or vibration (including the mirror movement in the camera) will cause
blur.

The speed of the film won't make that much difference between about 25 and 400
since the camera will adjust the f-stop. The finer grain of slower speed film
is wasted when the monitor has a maximum resolution of around 72 dots/inch.
Some types of *slide* film have a slightly different color balance, notably
Ektachrome and Fujichrome -- but in general, this is a waste of time since
the adjustment of the monitor will have much (several orders of magnitude)
greater effect. I would suggest 100 speed slide or print film.

Turn off the room lights or use a hood. Reflections will cause problems.

See what arpeture the camera picks and then shoot additional picutres at
-1, -2, +1 and +2 f-stops for slides or at -2 and +2 for prints. After a few
test rolls, use the setting that works the best in the future. The slides
will show you reasonably true colors. The prints are another story (see below).
In both cases, you may want to use an ultraviolet filter over the lens
to avoid color distortion be light you can't see.

For slides, the colors are as they are taken (with any ultraviolet distortion).
For prints, the colors are usually screwed up by the processing lab. The
problem is that the automatic  print making machines are set to balance the
color to an average gray. Whenever you have a picture with a solid background
that is not gray, you get what is called "subject failure." That means that
the picture itself (unlike most snapshots) does not balance to average gray.
For example, if the background is blue, the print will be too yellow to
attempt to make it balance to average gray.

Your best bet is to use a local lab and tell them that the roll of print 
film has "subject failure" and that they should lock the machine at the
base setting for the film type when printing instead of letting it pick
the color balance. (The base setting is the setting that would be used for
an exactly average snapshot that already balances exactly to gray. The 
base setting does not cause distortion.) 

Good luck!

Jim Winer
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