[pure-silver] Re: ZONE VI VC HEAD, was No More Below the Lens Polymax Filter Kits?

mail1 wrote (in part):


Cold cathode mercury tube variable contrast heads work on the principle of
adding blue light to the green light to increase the contrast. Usually the
green light output does not vary, an example of this is the Aristo vcl4500.
With my Zone VI VC head (a 4x5 model), I can adjust the green and blue tubes separately. 


Some VCL4500 units have a rheostat to adjust the green light level to reach
the higher contrasts. The Zone VI also includes variable green and blue
light. The problem is how to decrease the green light as the blue light is
increased so that the light intensity stays the same so that the exposure
does not have to be adjusted with each contrast adjustment.
Since the Zone VI VC head is so dim, what I do is run either the green or the blue tube at maximum intensity so as to get as much light out of the unit as possible. 

So for grades 00, 0, 1, and 2, I have the green at maximum.
For grades 2, 3, 4, and (wishful thinking) 5, I have the blue light at maximum. I calibrated things with a step tablet and a photometer so that Zone V always matches the grey card, using a constant 22 second exposure time. This results in apertures of f/8 for the extreme contrasts up to f/11 for grade 2. Fortunately, my enlarging lenses can have their click-stops turned off. So part of my chart looks like this: 

GRADE   SOFT    HARD    F/
00      MAX     OFF     8
0       MAX     B-      10
1       MAX     F       10
2       MAX     MAX     11
3       E       MAX     10
4       MIN     MAX     9
5       OFF     MAX     8
This is for Ilford MGIV VC paper in my version of D-72 developer 1+2 using a Kodak T-14 step wedge in my enlarger's negative holder for an 11x14 enlargement. 


This problem is
corrected better with incandescent light with filters. Dichotic filters
usually require some adjustment to exposure time. Some of the Durst Dichroic
heads have the ability to introduce neutral density to adjust for this
problem.





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