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Contact Sheet/Callier Effect?



 
 
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Old May 13th 11, 03:53 AM posted to rec.photo.darkroom
Jean-David Beyer
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Default Contact Sheet/Callier Effect?

Len wrote (in part):

It is often stated that if you want your prints to have similar
tonal values to your contact prints you need to try diffusion or cold
light as your illumination source. I use a condenser enlarger and I
have noticed a change in values from the contact sheet to the print
even before I was aware of the different light sources and the Callier
effect . I have always just attributed the difference to the visual
effects of scale-up. Medium format to 8x10 or 11x14
.
I do know this though: I have never been fully happy with my tonal
values, especially the highlights. In fact between this and the very
noticale dry-down effects of Illford fiber based paper it drives me
nuts.

In making my contact sheets I place the negatives on the paper under a
sheet of glass. In the enlarger I have the 2 1/4 in film holder. I
focus the light so that the edges of the (empty) film holder are sharp
at he easel. So, the enlarger is focused and at a height very much as
it would be if I was making a print.

Back to my question. Since this is a condenser enlarger is this light
still collimated or not when it strikes the paper after traveling
throug the lens? Is the Callier effect at work when I make a contact
sheets or not? Please explain.

Actually, it does not make any difference if the light is collimated
when you are making contact prints. Where it matters is when the
distance between the emulsion and the enlarging lens is large, such as
between the negative and enlarging lens in a typical enlarger in normal
use. This is because the collimated light is scattered by the grains in
the negative, and the scattered light pretty much does not make it to
the lens.

Where scattering might make a difference when making contact prints is
when the distance between the emulsion in the film is distant from the
printing paper. The typical contact printing arrangement is such that
this distance is extremely small. So even if you used a point source of
light through perfectly focused condenser lenses onto the entry pupil of
the enlarging lens at one extreme, and a cold light diffusion source at
the other, you would see no change (other than exposure difference if
they sources had different useful intensity).

The difference would be noticed in the enlarging situations, as Richard
has already pointed out.

Now I have used both semi-collimate light from a normal enlarging bulb
through Beseler condenser lenses, and also Aristo and Zone VI cold light
heads and prefer cold light heads (for B&W). But satisfactory prints can
be obtained with either. If you calibrate your exposure system and
negative development, you will get the same results. Richard says so.
Dr. Richard J. Henry, who really ought to know, and I say so.

The claims of preference of one type vs another seem mostly from people
who have not really calibrated their exposure and development to give
the same print contrast.

The main advantage to cold light enlargement is that the negatives have
less of a chance to pop from the heat. Since I use a Beseler
neg-a-stretch holder, they do not pop, although some people call that
holder a neg-a-scratch holder. I never compose that close to the edge,
so it does not matter to me.

Another claim, that I do not find matters to me; i.e., I cannot
duplicate it, is that dust is less visible from prints using diffused
light. I do not see the improvement. I can see dust no matter what kind
of light I use. And since I am a hopeless print spotter, all I can do is
toss the print, clean the negative again, and make a new print. I have a
Zone VI high voltage electrozapper brush that works very well on 4"x5"
negatives. Not quite so well on 35mm ones (brush is too big). I got one
long ago. The last time I saw the price for one, I thought it was too high.

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