From: donl@glass.esd.sgi.com (donl mathis)
Newsgroups: rec.photo
Subject: Re: Enlarger light sources
Date: 23 Jul 92 00:19:32 GMT
Reply-To: donl@glass.esd.sgi.com (donl mathis)
Organization: Silicon Graphics, Inc., Mountain View, CA, USA


In article <1363@sousa.ltn.dec.com>, madler@vssad.enet.dec.com (Michael
C. Adler) writes:
|> 
|> I'll let my curiosity get the better of me and probably start a
|> religious war.

One can only hope.  We need a little excitement around here.

|> What are the fundamental differences in both B+W
|> and color enlargements made with condensor, diffusion and cold
|> light sources?

You've heard about the contrast difference between diffuse and
condenser light sources; it's very real.  Condenser light is
collimated, i.e.  unscattered, nicely arranged rays, and silver grains
tend to scatter it.  The denser the emulsion, the more the scattering,
so it's the highlights that are affected the most.  What happens is the
light that should have hit the paper goes somewhere else, so even less
light hits the paper, and the highlight contrast increases.  Below a
certain density, there appears to be no affect whatsoever, so the
middle and lower tones all stay just as they were.  The net result is
that the overall reproduction curve gets a bit of a kick in the
highlights.

Diffuse light gives a more accurate, linear representation of the
densities in the negative.  It also tends to soften the edges of the
grain and other sharp edges in the path, such as dust and scratches.
Edge contrast is not resolution, though at times it *looks* like it.
It's not clear to me whether there is a minor drop in overall resolving
power, but micro-contrast at edges appears to be reduced, which
certainly leads in that direction.  The real test of whether this is a
problem or not, of course, is the print, and I can't say I've ever been
disappointed in a print's sharpness or resolution because it was
printed with diffusion rather than condenser light.  In fact, my
favorite photographs in all the world were made with a diffuse light
source, and to me, they look mighty sharp, perhaps amazingly so.

|> I've read that many color heads use diffusion.  Is this simply a
|> space constraint or is there a more sophisticated reason?

Well, consider that color is usually made either by mixing three
different light sources, or putting three filters into the path of a
single light source, and then mixing it all up.  In either case, you
really need a chamber with a white interior where you can mix the light
into a nice, even color.  Since a condenser's job is to focus the light
source onto the aperture of the enlarging lens (if I have my optics
right -- they're at least close), it's a bit tough to do that with a
mixed light source.

Plus, diffuse light makes better prints.  :)

|> Cold source advocates claim that these lights actually decrease
|> grain.  Given that grain is effectively the image how is it possible
|> to decrease grain without sacrificing sharpness?

I think I blathered about this a paragraph or two ago.

|> Is the color temperature (or light spectrum) of cold sources suitable
|> for positive and negative color enlargements?

Ick.  Ew.  Ptew.  Cold light tends to have nasty sharp peaks at
specific frequencies -- very much an obstacle to ration control of the
color balance.  I suppose it's possible, and I know people have done
it, but I never would.  A good-old tungsten light source gives a smooth
spectrum that can easily be controlled with the filters.  A good color
head is really the best way to do color.  If money were really a
problem, I would try and find a diffuse tungsten black and white head
that takes color filters.

- donl mathis at Silicon Graphics Computer Systems, Mountain View, CA
donl@sgi.com

"We're seeing Buicks the size of prominences!"