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Old June 5th 09, 11:04 AM posted to rec.photo.darkroom
Jean-David Beyer
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Default Compatible Cold light heads for Omega D5V

Richard Knoppow wrote:
"Lew" wrote in message
...
Can anyone give me a run down of what's available, what to look at &
what to avoid? Thanks.


Since others have given you some answers I will ask a question: Why do
you want to change to a cold-light head?

Richard's implied suggestion is a wise one. All manner of magical properties
are ascribed to cold light heads.

The most important one is supposedly that you can use it to print negatives
that are unprintable otherwise because their contrast is too great. Of
course, that should seldom arise because you calibrated your exposure,
developing, etc., to your existing enlarging system already, right? So if
your negatives have excessive contrast to print even on the softest grade
paper, you mane a serious mistake in exposure, or development. Picker seemed
to feel that exposing and developing for (IIRC) grade 3 paper and printing
with a cold light gave better quality prints than exposing and developing
for grade two paper and printing with a condenser head. But as Dr.Richard J.
Henry proved, the two will have identical D/H curves. So this argument turns
out to be of no validity. A cold light head will be helpful in controlling
contrast only for those who overdevelop their negatives. It was my
impression that people used to underexpose their negatives (they thought
they were increasing the speed of the film), and over develop them to
maintain contrast in the mid tones (only to get burned out highlights). But
if you calibrate your film, you will get the best speed of which the film is
capable (but not 800 from Tri-X; probably not even 400).

The next most important claim is that with diffuse illumination, dust and
other defects of the negatives simply disappear. This turns out to be
nonsense. Picker's most important cure for this problem is not his (or
anyone else's) cold light head, but his negative cleaning brush, the one
with a 5000 volt neon sign transformer that produces a corona near the
bristles of the brush with which you wipe the dust off the negative. A cold
light head lowers the contrast of your print by about one contrast grade
compared with what you would get if you used a condenser head. But then you
have to use a grade harder than the condenser head required to get the
proper contrast, and the dust and scratches are back. Sorry, the only good
solution is to keep your negatives clean, or to clean them.

As far as I can tell, the only reason to use a cold light head is that it is
somewhat cooler than the typical condenser head. This can reduce the risk of
the negative's popping between when you focus it on your paper and when you
actually make the print. I have never had this happen, but I do not doubt it
can happen, especially with small negatives (e.g., 35mm). It does not happen
for me with 4x5" negatives because I use Beseler's Negaflat (I call it their
nega-stretch) film carrier there that puts the film under slight tension so
it must be flat. Others call it their nega-scratch carrier because it tends
to scratch the extreme edges of the negative. I do not compose my images
quite that close to the edge of the image area, so it does not bother me.
For other sizes, you might consider using glass holders (I forget what they
are really called: the ones where the film is held between two pieces of
glass) that should also keep them flat.

If you get a cold light head with two tubes, a blue one and a green one, you
will not need to bother with variable contrast filters to get the contrast
grade you need (if you use variable contrast paper). You just dial in the
amount of blue and green you need to get the contrast you want. I find this
a convenience, but a color head would do the same thing. Aristo still make
these, and Zone VI used to. The main troubles with the Zone VI are that the
thing is not really bright enough, though it is definitely usable; and that
the green tube has to be warmed up considerably to get the regulator to keep
it stable, which is annoying. I hope that the Aristo does not suffer from
this problem.

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