Well, after overrunning lunch by 2 hours because I was fixing a production-line test rig (the boys have to come for Sat morning overtime to catch up now), I didn't have time to test an orange filter anyway, so I took it off and just ran unfiltered. Still as cloudy as all hell out there, same ev9 reading, same 8 mins, I'll see what I get out of it when I dev it tomorrrow morning (no darkroom tonight, seeing John Zorn)
Yep, ev9 gave me 8 minutes at EI6 for my very first 10x8 pinhole the other day (mid morning, on MGIV RC) and gave a very acceptable image indeed.
Now of course I've run out of MGIV and have a selection of very old graded papers to play with, each of which will require calibration ... sigh
Sigh indeed. I got home on friday and realised I'd left the pack of MGiv paper open. Tested one and there's a strip of black down one edge, the one from the top of the stack is probably a goner.
so, more updates. The 8x10 I shot friday turned out very very black. But not so black from a light leak, there were a few spots of white from some deep shadows. Scanning the hell out of it I got some kind of image, but I definitely need to rate it a stop or two faster than the ei3 when I run with no filter.
Today I took it out again, and this time I found a B+W 52E 2x Yellow-Green filter to put on it. The clouds are still around, but the sun was trying to break through a bit. EV10 at ei6, minus the 7 stops for aperture gave me 2 mins so I went with that (ie, it was ei6 including filter factor, or ei12 minus one stop for the filter).
I got something almost repectable, a little thin and the shadows look totally white, but better than all black. So maybe ei3 including filter factor for the 52E might be the way to go.
I've also bought a different step ring so I can mount a 39mm real Yellow filter on it. I've just looked to see what this one is and all it says is "Ernst Leitz GmbH Wetzlar" and "1" and "Germany" on it. I've never noticed that it doesn't have a filter factor printed on it because it lives on my TTL Bessa. So once I try this one I'll have to experiment again.
Even with the filters, it may be a challenge to get those clouds and also get shadow detail.
I rate MGIV at ISO 6 when not flashed, so you are very close. Close enough that remaining differences can be in how you meter and how you develop and different errors in calculating f/ratios... I still think even a yellow-green will have a higher filter factor for paper than for panchromatic film....
May I just point out that paper is not panchromatic. The use of a dark orange filter will be turning your exposure in to a sort of semi-safelight exposure, and also reducing the transmitted green light used for the low-contrast multigrade exposure (as well as the blue light too of course). Also note that daylight is relatively blue compared to the light temperature coming out of the enlarger (for which the papers are designed) and that it is the additive effect of the blue-sensitive parts of the paper emulsions which produce the high-contrast, so it has a potentially larger share of the exposure in an unfiltered pinhole camera - to be clearer, you don't end up with a grade-2 middling contrast grade if you use no filter in daylight. Precisely what you do end up with depends a bit on what paper you have, the time of day etc.
Assuming the paper is multigrade, then use of yellow-ish filtration might help control the contrast slightly, at the expense of more exposure. You can also try a pre-flash, to raise the shadow exposure a little, and then adjust exposure to suit your highlights.
Check that your pinhole is the "optimum" size (should that really be f344?) and then base your exposure on that value, a light reading and the Ilford-Harman pinhole exposure slide-rule.
This is what I find if it is still relative.Attachment 90738