I finally made it into the darkroom and figured out my preflash: 1/3s f/22 full-height on the spare enlarger. 1/2s gave the tiniest bit of tone after drydown but 1/3s was clear. Also discovered the club's safelights aren't very.
I found that preflash has a visible effect but the reverse of what I wanted! I only tried it with white/unfiltered light from an enlarger and didn't get around to preflashing magenta.
It seems that if I preflash, I can get detail in the highlights at a higher print grade but the contrast index in the highlights is lower. In other words, it seems to roll the curve off at the highlight end, which is what you'd expect given that you're adding a bunch of non-image light. In the highlights, the image is largely made up of preflash (and without preflash they'd be blank) whereas in the shadows, the preflash has a very small contribution to the exposure. Say I have a scene I could fit into the paper's dynamic range at grade 2 (but it looks flat) or at grade 3 and lose the highlights; with preflashing I can print it at grade 3, get more CI in the shadows and still hold detail in the highlights, but the highlights will be closer to grade 1.5. So it's definitely a useful technique that I will be exploring heavily with some high-key photos. I think it should also be good for reducing grain in skies and I have an image I think it will work wonders on next week for that purpose.
I tried printing with less-yellow light (120Y instead of 200Y) for the low-contrast part; it made no difference. I think that's a function of how my colour head works - say I wind in 100Y, the position of the dichroic filter means that blue is blocked from 1/2 of the flux from the bulb. So I seem to get exactly the same image whether I use (say) 10s of 200M and 4s of 200Y as if I used 8s of 200M and 4s of 100Y. If I was using discrete filters in a B&W head, this approach (say, print with 1 and 4 not 00 and 5) may well be valuable, I don't know.
I also tried yellow-first split-grade but found that I got results much flatter than I wanted. Typical exposures I'm using without preflash are between 4:1 and 8:1 M:Y, so by the time I have visible tone from yellow exposure, there's way too much of it. Might be I'm doing wrong, I dunno - I've only been printing for about 6mo. I think I might just need to pay more attention to drydown. And I think I'm definitely getting shoulder effects in some of my shots, so I'll need to pay a bit more attention there - seems particularly problematic when pulling Pan-F.
Thanks for all the suggestions, they're appreciated. If I had a flatbed scanner I'd post the results, but my only digitisers are a DSLR and film scanner.
