Converting single filter exposure to split filter exposures?
How do I easiest test/obtain what split filter exposures in percentages or, more practically, in stops, are equivalent to an exposure with a certain contrast filter?
At hand I have step wedges, both transmission and reflective, from Stouffer (TP4x5_31, TP35_21, R2110), An Ilford E-10 exposure meter, Ilford Multigrade filters (under the lens), A B/W densitometer (for both transmission and reflective).
This would simplify and speed up my darkroom work quite a bit.
I've calibrated my paper/filter combination so that I can find Highlight exposure with a single filter, with a test strip and changing exposure only. By applying my calibration data for exposure changes to keeping highlights constant, I can then find the contrast filter to give proper shadows with a test strip where the contrast filter is changed.
Since different areas of a print often calls for different contrast, the final print is easiest implemented by split filter printing with different exposures for different parts.
I could of course make two test strips also here. One for the highlights using the soft filter and another one for the shadows using the determined highlight soft filter exposure plus a changed high contrast filter exposure. This way I can achieve a higher precision of the shadows since the contrast changes are not discretized by the half grades of the contrast filters but can be changed in arbitrarily small steps by changing the relation of highlight exposure to shadow exposure. However, I've found that the need for shadow precision is smaller than for highlight precision and the half grades that my multigrade filter kit provides are sufficient precision. More Importantly, I prefer to make localised test strips, i.e., I prefer to test the same area of the print for each step in the test strip by sliding the paper under a mask centered on the area of interest, in stead of making moving test strips (the traditional type) where each step in the test strip is from an area adjacent to another one in the print.
The later traditional type is more convenient when making two exposure split-filter test strips for the shadows, since you can make the soft filter highlight exposure for the whole strip and then the steps by changing the high-contrast filter exposure, while for former, the localized test strip, you have to make two exposures with a filter change in between for each step as the paper is moved in between. However the former, localized test strip gives more precision since it tests the same area and is easier used with f-stop timing.
Consequently, what I want to be able to do is to make localized test strips using my calibration data to find a single filter exposure and a filter number for different parts of my image. Then for each part of my image I want convert that exposure and filter number to a split filter exposure pair that would be equivalent. Then I can calculate a base split filter exposure pair for the whole image and burn in different parts with the remaining exposure times for the high and low contrast exposures that will be different for the different parts of the image.
The only thing I need is the data for how to convert a single exposure and filter number to a split grade exposure pair. Can I obtain this in a simpler way than making a split filter test strip for each under-the-lens filter number?
I'll apprecieate any help, links, reflections, thoughts etc. The wonderful book "Way beyond Monochrome" by Lambrecht and Woodhouse have helped me and inspired me in the darkroom a lot (I warmly recommend it) but does unfortunately not (un)cover my little problem.