Keith Schreiber (jkschreiber@worldnet.att.net)
Fri, 02 Apr 1999 01:36:26 -0700
Charles,
Thanks for setting me straight. ;-} Please note that I've changed the
subject line to reflect the tangent we are now on.
>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.
As good as, but still different than, n'est pas? My, um, point was that the
NuArc and other similar units are not accurately called point sources. Are
they?
>> 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).
I've never used mylar or anything else between film and paper, though I can
certainly imagine how this could be a problem with a diffuse light source.
Of course, it could also be used intentionally to achieve that effect.
Perhaps with your own work the difference is as noticable as you indicate.
With mine it is not. I print Pt/Pd (well, mostly Pd) and use a variety of
papers with a variety of surface characteristics. The prints to which I was
refering were both made in a vacuum frame, one with the NuArc and one with a
BL lightbank, on Simili Japonwhich has one of the smoother and harder
surfaces I've printed on. Perhaps the use of the vacuum frame is more
significant in this case than the light itself. I have found contact
problems on occasion with the use of spring back contact frames or - even
worse - the plate glass sandwich that I have used for printing
multi-negative panoramic pieces.
Any chance you will be at APIS this summer? Perhaps a little show-and-tell
would be interesting.
>> 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).
Okay, okay. Could you demonstrate that somehow, or maybe provide a reference
with diagrams. I really am interested. What does "subtend" mean? At any
rate, doesn't it vary with the size of the image, distance from source,
etc.?
Regards,
Keith
Keith Schreiber
jkschreiber@worldnet.att.net
This archive was generated by hypermail 2.0b3 on Thu Oct 28 1999 - 21:39:29