Becky,
I pre-flash before each use. I have, however, pre-flashed a whole batch for my 8x10 pinhole box camera with the storage compartment, then went on a week-long vacation, and have also done so with the plywood 4x5 while on vacation, and saw no difference in the image. I suspect the latent preflash image would last a long time, as long as any latent exposure on paper will last.
I use the same preflash amount for the same paper, regardless of which type of camera it's going into. For the grade 2 Arista RC paper I preflash for around 8 seconds, while for Harman DPP it's 3.5 seconds. This makes sense because I've seen little or no reciprocity effect with paper negatives, so there should be little difference between a sub-1 second glass lens exposure and a lengthy pinhole exposure, in terms of how the preflash effect works with the in-camera exposure.
If a person wanted to get real scientific about it they could use a different preflash amount for indoor studio lighting versus daylight, since in the studio you can control the color of light and hence affect the contrast artificially, but you'd have to experiment with it first. For my still lifes, I just use indirect north-facing window light and I get good results.
I think it's the shade of gray that matters. I touched on this in the video but didn't expound on the reasons, but each step in the zone system is a doubling or halving of the exposure from its neighbor, so that if I give a preflash exposure to paper that brings the amount of gray up to, say, zone 1, and there are 9-1 = 8 zones between zone 1 shadows and zone 9 highlights, the amount of highlight exposure is therefore 2^8 = 256 times more exposure than that zone 1 preflash exposure, so therefore that little zone 1 preflash exposure will hardly affect at all the highlights, which has the effect of reducing the contrast.
I'll try to remember to include in my next video a shot of what the light looks like when in operation. In general, the light casts a faint circle of light upon the work table in my darkroom that's maybe 3' in diameter, and I just make sure the paper is face up somewhere near the middle of that circle, and that there's nothing else that can reflect light and mess things up (like the piece of contact printing glass to the right of the gray cutting board in the video, for instance, that could cause some weird effect due to reflection of light). The 5mm aperture in the light source, plus the height above the work surface, is how I'm controlling the strength of the preflash light. My desire was to get it weak enough so my exposure times would be in the 5-10 second range, sufficiently long to be accurately timed via my old Gralab timer.
I should have included it this time, but of course the director (me) forgot to tell the cameraman (also me)! It might also help if I scripted or storyboarded these videos a little more thoroughly, as I tend to work from stream-of-consciousness a lot, then edit afterwards.
~Joe
