Tom Ferguson (tomf2468@pipeline.com)
Sun, 07 Mar 1999 08:42:21 -0800
>Judy Seigel <jseigel@panix.com> wtote:
>Another possibility is that the theory doesn't apply. I had many an
>airhead undergraduate who didn't know that variable contrast is a special
>formulation & used to "correct" contrast of lith film with the VC
>filters, until I explained, no you can't do that.
>
>A few SWORE that it did so work, until I explained that that was just the
>change in exposure, less exposure, less contrast. They either believed me
>or pretended to. But now I'm thinking, it is possible that colored light
>did do something to the contrast... a gray area, so to speak. Why not try
>it, we'll never tell.
What we are seeing here may be more than contrast change due to
underexposure. I suspect that we are seeing reciprocity failure/effect.
I've found that beyond 15 seconds, APH film changes contrast significantly
with increased exposure time. 15 seconds at F8 is NOT the same as 1 minute
at F16! I've settled on one "standard" time, and vary exposure by F-stop.
If I remember correctly, the higher numbered variable contrast filters also
have a higher filter factor?? It has been a long time since I used these!
If so, the students may be using the higher contrast filters, and thus
increase their exposure time, and thus get reciprocity failure, and thus
get more contrast. If I'm correct, it would be a lot easier to simply stop
the lens down ;-)
Tom
tomf2468@pipeline.com
http://www.thefstop.com/tf.html
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