Bruce L. Bevelheimer (bruceb@ricochet.net)
Thu, 01 Apr 1999 22:53:19 -0800
As an amateur astronomer, I'd like to offer a minor correction. The
angular diameter of the Sun as seen from the earth averages about HALF a
degree. In fact, checking the diameter as I type this (via software
calculations) shows a current diameter of 32.01 arcminutes -- better than
the
carbon-arc plate burner.
Bruce
-----Original Message-----
From: Charles Steinmetz <csteinmetz@redneck.efga.org>
To: alt-photo-process-l@skyway.usask.ca
<alt-photo-process-l@skyway.usask.ca>
Date: Thursday, April 01, 1999 9:26 PM
Subject: Re: Trials and Tribulations of the Digital Neg.
>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