From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 06/20/01-04:32:40 AM Z
At 03:30 PM 06/19/2001 -0600, you wrote:
Snipping.....>
>Dr. Henry in his famous book on B&W printing, whose full name and book
>title escapes me at the present, poo poo'd the idea of diffused enlarging
>printing saying it is just softer in contrast. A.A. disagreed. Some
>printers swear by diffused enlarging and others swear by point source. The
>same is true of contact printing.
>
More snipping
_Controls in Black-and-White Photography_ Second Edition, Richard J.
Henry, 1986, Butterworth Publishers--The Focal Press ISBN 0 240 51788 1
Long out of print.
Dr. Henry did not just poo-poo this, he proved it. He shows reflection
density measurements of The same negatives printed on both kinds of
enlargers but with paper contrast adjusted. And of negatives adjusted in
contrast to match the light sources printed on the same contast paper. In
both cases the curves fall exactly on top of each other.
I have both the original condenser head and a cold light head for my
Omega D2V. After experimenting around I found I could also get identical
results with both. I didn't do the controlled sort of testing Dr. Henry did
but made visually matching prints, which is, after all, about all one can
ask.
I reverted to the condenser head eventually. It has more light output for
small format and is actually more uniform for 4x5.
I have seen no experimental evidence comparing the effects of contact
printing with diffuse vs: specular light. It would appear from recent
postings to this list that several list members have done this.
The difference in contrast in enlarging is due to light scattering by the
particals making up the image (Callier effect). The ratio of contrast
depends on how much scattering there is. Coarse grain, thick emulsions have
a lot of scattering and hense a lot of change in contrast. Fine grain and
thin emulsion films have less scattering and the dye images of color
negatives or transparencies have virtually none. Probably ink jet images
have little or none.
If my understanding is right (and it could well be wrong) the Callier
effect disappears where the negative is in contact with the printing
material since all the light striking the iamge (except the small amount
reflected) is transmitted to the printing material where it is not where it
must progress to a relatively distant lens.
It should be easy to prove this one way or the other with conventional
materials being careful that there is not some other difference, like a
change in the spectral characteristic of the source, going on
simultaneously.
---- Richard Knoppow Los Angeles,Ca. dickburk@ix.netcom.com
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