Here's a digital picture of my new densitometer that I just built cost about a buck.
I'm using this for using the zone system in my darkroom. I just made a H&D plot of Regent Royal ortho-litho film developed in D-23 1:3 for six minutes. It goes all the way up to a density of 4.5. This requires a range of about 60,000 to one for the measurement range, not all that easy to do. But my meter works for this now.
My densitometer/ darkroom meter is a 5mm diameter cadmium sulfide photo resistive cell that I bought on Amazon in a 30 for $5.99 bag, or 20 cents each. Add four #10 machine screws, nuts, and washers and three pieces of 4x4 inch 1/8 in. thick mdf for a total of just under a buck. This is read out in ohms on the DMM I already owned, which has a hold button, almost a necessity in the darkroom.
For use, I need to calculate the conductivity of the cells by finding the inverse of the resistance, then multiplying by a large number to give arbitrary units for fining ratios. I do this by storing 10,000,000 in calculator memory, then dividing the memory read by the resistance in ohms. This gives reading ranging from 1 to a few hundred thousand since my meter reads a maximum of 10 megohms.
Liam Lawless wrote articles on making enlarged negatives using ortho litho film reversal processing in a peculiar way that includes an extremely healthy flash exposure about 13 years ago. His article does not give a theoretical description of the process, but rather a trial-and-error method that's pretty complicated. This has been discussed in this forum in the past. I assume digital negatives killed any interest in this.
I will be using this process to enlarge my 35mm and 4x5 negatives for alternate process printing. By understanding how this process works, and metering the original negatives, I will be able to produce enlarged negatives of almost any density range from negatives with variable ranges and get a good result on this first try. This is what I call zone system in the darkroom.
Ortho-litho films have short toes, followed by a short and steep linear area, which starts rounding down, followed by an enormous, rounded shoulder. At the top of the density range, the response is pretty linear and low contrast. This is why this process works. A density curve is needed to calculate both the negative exposure time and the flashing time. At the top of the scale, we find the exposure that gives maximum density and then the exposure that gives our desired density range subtracted from that maximum density. That will be close to the negative exposure time. We then push these up the graph scale with a flash exposure calculated to give the maximum density with the negative exposure time.
I will also use my Devtek processing drums, which is much simpler than the seven or eight trays you would need otherwise. I will need to build a light saber to stick down into the tube for re-exposure.
Comments and suggestions welcome.
Alan Townsend
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