Adapting print recipes.

Today I needed to reprint five prints at a different size. They were 7.5 x 7.5 and I needed to print them 10.5 x 10.5

I had good documentation for the original size so using the capabilities of my Zone VI stabilizer and RH Designs Zonemaster II, I was able to set the light intensity at the new height to match that of the lower height. I thought that was pretty cool.

Trouble was, I had to make the new prints on MGIV and the originals were on Kentmere Fineprint. It turns out that not only is the (with my configuration) Ilford a slower emulsion than the Kentmere...it was also harder.

Some testing allowed me to get the best match between the original and new print by giving the MGIV 3/4 stop more soft exposure and 1/2 stop more hard exposure.

As it turned out, this ratio worked for all five prints though the original exposures were all very different from one another and the original ratios for soft and hard exposure varied as well. Whatever the original print's soft and hard exposures were on the Kentmere, 3/4 and 1/2 stop more respectively, duplicated it (to the extent that the paper's differing characteristics allow duplication) on the Ilford.

This is extremely useful information for me.

What other strategies do people have for adapting a print recipes when altering one or more of the variables?

Some info is the speed of the paper

Buried in Ilford, Agfa, and I thing Kodak's pagakging of B&W paper are little leaflets that tell you about contrast ranges in the negative that correspond to the multgrade filter for that denisty, and usually in that table there is a suggestion of the relative speed of the paper under raw white light, as well as under the filtered light.

I believe there is even a scale on an old kodak darkroom dataguide that I have that helps out in switching between papers. I don't recall what the units are called. It is on the inside if the scale that I usually use to calculate new exposure times when raising the enlarger head after making a modest sized good test print.

I do recall that Ilford MGIV is something like 400 under white light, while Agfa MG?? 111 is like 250, i.e it overall needs 400/250 worth of the Ilford light to produce the same level of blacks

The adjusting of filtration might be discernable from the contrast rnage data, but I think that there will still be some trial and error work reuired when swapping papers.

Fred Picker wrote about using his stabilizer to vary exposures from one paper to another. IIRC, he'd have to find the right percentage by trial and error, but then he'd write it down in his lab book. He described the process in his newsletters.

Finding the calculation for different print sizes is rather easy, but it seems to me that when changing papers, there's simply too much subtle variation to rely on a formula. I just use my eye and consider a switch to a different paper as making an entirely new print.
juan