----- Original Message -----
From: Mafud
Subject: Re: RC Papers for cold-head enlargers
> In a message dated 2/25/01 5:06:26 PM Eastern Standard Time,
> [EMAIL PROTECTED] writes:
>
> << I am not in agreement that _all_ tungsten powered enlarger
sources are
> prone to uneven illumination.
> Thanks
>
> Bill >>
>
> At the lab I belong to, our membership and lab fees pay for
maintenance of
> all lab equipment and processing supplies.
>
> We were instructed in proper enlarger alignment, but usually
the topic is
> never raised in practice. I understand proper alignment has
much to do with
> accurate printing and controlling "hot spots."
> I'm getting a free "clinic" with this thread.
>
Here a bit more, just to further mudify the muzzified. Or at
least, my thoughts on hot spots and the like. The alignment you
were likely taught is the relationship between the easel, lens
and negative carrier. This won't have any effect on curing
uneven illumination, but will certainly help cure corner to
corner sharpness.
Hot spots are generally a problem with poorly set up condenser
systems, and there are a variety of ways to combat it. Beseler
uses a double bellows system, whereby the lightsource can be
raised or lowered in relationship to the negative. This allows
for focussing the light source onto the negative. Both Omega and
Durst use additional condensers to focus the light onto smaller
negatives.
If the light isn't focused on the negative, then a hot spot
will result. This is because there are two optical systems at
work in a condenser enlarger, one above the negative (the
condensers), and one below (the lens itself). They both have to
be focused on the negative to avoid hot spots. I suspect that
many of the problems with condenser light sources are caused by
poor condenser focusing.
So, enter the "cure", the diffuser enlarger. They come in a few
flavours, but generally are either a dichroic light source (di=
two, chroic=colour, Latin, I think), or "coldlight".
Coldlights are not really cold, but they are certainly not
hot either. They have an operating temperature, and if they are
run below or above that temperature, the quantity of light they
produce is not predictable. They are also an inductive light,
and are pretty hard on timers, unless they are set up
specifically to handle the load. I burned out a Gra-Lab model
900 with my coldlight before I bought the Zone VI timer.
As Ed mentioned, there has been a lot of technology put into
making coldlights more stable. This seems to involve putting
small heaters into the head to warm everything up to what is
close to operating temperature. If the lamp starts at operating
temperature, then the amount of light generated during exposure
will, in theory, be repeatable. There are a variety of second
party methods used to stabilize coldlights, but for myself, I
think Zone VI got it right with the compensating darkroom timer.
This little gem of a device uses a photocell in the head to
measure the light output from the tube, and compensates the
exposure time accordingly. No attempt is made to alter the
amount of light the head outputs with this unit. This allows the
light to run at it's full throttle if desired (there is also a
means for turning the brightness down, and also a "drydown"
compensation on the unit). The advantage over conventional
stabilizers is that they universally run the tube at less than
full output, thereby increasing exposure times. Any coldlight
with a built in stabilizer will be operating this way.
The downside of coldlights is that they are prone to uneven
illumination. Not "hot spots" per se, but noticeable unevenness.
This is caused by the fact that coldlights are a neon tube bent
into a shape that will fit inside the head. The light is dimmer
at the bends, and this, combined with any irregularities in the
diffuser plastic or reflective problems within the head causes
problems. Zone VI made a couple of cold lights that had the
bends outside of the negative stage area to cure this problem.
One was designed for graded paper, the other for VC paper, and
used two tubes one blue, one green for controlling contrast. I
don't know if they are still available, Calumet is the
distributor for Zone VI products now. I recall they were really
expensive.
Probably the best all round light source for B&W is the
lowly dichroic head designed for colour work. It has the
advantages of being a diffuser light source, which is much
easier to work with, and by using the built in filtration, very
fine control of variable contrast papers is possible. Generally,
the illumination from them is very even, unless there is
something grossly wrong with the light source. I am of the
opinion that modern black and white emulsions are optimized for
diffusion light sources.
Thanks
William Robb
-
This message is from the Pentax-Discuss Mail List. To unsubscribe,
go to http://www.pdml.net and follow the directions. Don't forget to
visit the Pentax Users' Gallery at http://pug.komkon.org .
