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When you change paper grade the exposure change you need depends on the gray tone you wish to hold constant. If you are changing from grade 3 to grade 4 with MGIV RC and wish to hold the highlight tone then you need to increase exposure by 1.2 stops; if you wish to hold black constant then you need to increase exposure by 0.4 stops.
The paper speed tables on the Darkroom Automation web site can be used to determine the exposure change. The one for MGIV RC is located at:
Go to cell A64 for the speed table. The rest of the spread sheet holds the raw data and the paper's HD and Zone speed curves. These speed charts are for use with Ilford's filters. VC and color heads have so much variation between them, and so many heads exist, that providing speed data for all the variations is beyond Darkroom Automation's abilities. The Quatro sheet allows you to enter your own raw data and generate a speed table and set of HD curves; the needed mathematical functions are lacking from Microsoft's Excel.
Paper speed, in the Darkroom Automation world, is the amount of exposure needed to produce a given tone. Darkroom Automation measures time in stops (0 = 1 second, 1 = 2 seconds, 2 = 4 seconds...) and illumination in the reading of the Darkroom Automation meter. The total exposure is the meter reading added to the timer setting.
The Darkroom Automation system is calibrated so that an exposure of roughly 10 stops will produce black on Ilford MGIV RC with a #2 filter.
Speed charts for other papers are available in the support files section of the Darkroom Automation web site:
To find the exposure compensation needed when changing grades find the paper speeds for the tone of interest and subtract the two numbers: in the paper speed table, look across the row for the tone you want to hold constand; find the paper speed for the two grades you are using; and subtract the numbers. In the above example you go from grade 3 highlights with a paper speed of 6.03 (6.03 stops of exposure are needed to produce a highlight tone) to grade 4 with a highlight speed of 7.20 (7.20 stops of exposure are needed to produce a highlight tone) with a difference between the two exposures of 1.17 stops.
You don't need the Darkroom Automation meter or timer to use the tables. The data can be used without any metering at all when you need to know the amount of exposure change to make.
A table of stops<->time is available at: