Charles Steinmetz (csteinmetz@redneck.efga.org)
Fri, 02 Apr 1999 05:31:04 +0000
Keith wrote:
> Something along the lines of the NuArc 26-1K Mercury Vapor unit is
> usually considered a point source, at least in comparison with a
> fluorescent unit. Personally, I think this is a mis-characterization,
> since the NuArc unit, which has a vacuum base of 23x27 inches,
> utilizes a reflector of approximately 16x20 inches.
If the reflector is a good approximation of a parabola with the lamp
at the focus it will generate collimated light, which is just as good
as a point source.
> I would be most impressed by anyone who could look at pairs of prints,
> one from each light source, and be able to consistently identify which
> was which. That said, I want to point out that I use in-camera large-
> format negatives.
I make in-camera LF negs myself, usually of subjects with lots of fine
detail. I can easily tell the difference between point-source prints
and light-bank prints with gelatin POP, carbon, and Pt. (Bear in mind
that I coat Pt on the hardest, smoothest paper I can find and do not use
protective mylar between the negative and the paper. With protective
mylar, the difference is glaring -- nobody could miss it).
> And don't forget - the sun is the ultimate point-source.
Not really -- the sun subtends an angle of 3 or 4 degrees at the earth's
surface. A true point source should be more like one minute of angle.
A typical carbon-arc plate burner subtends less than one degree, three
or four times better than the sun (but still tens of times short of
being an ultimate point source).
Best regards,
Charles
This archive was generated by hypermail 2.0b3 on Thu Oct 28 1999 - 21:39:29