Sage advice. If there is one person here to trust regarding pinhole work, it's Tom. Testing rules, for sure.
My 'magic number' happens to coincide with my math above; I get the best from my pinhole camera at 1s in broad daylight. What prints well is what works best, all science aside.
Originally Posted by Tom Miller
With a light meter and a calculator you can determine the difference in stops required by the aperture number of the pinhole.
Suppose you’re using ASA 100 film and the meter reads 1/125 second at f/11. We suppose that you’re using an f/158 pinhole.
The formula is
Difference in stops = 2*ln(f1/f2)/ln2
The constant factor 2/ln2 = 2.885, so we can simplify the formula to
Difference in Stops = 2.885*ln(f1/f2)
Where f1 is the aperture number in the meter reading and f2 is that of the pinhole.
Then in the example
Difference = 2.885*ln(11/158) = -7.7 stops.
The light is reduced (hence the negative sign) by 7.7 stops.
So, disregarding reciprocity failure, you should increase the time by 8 stops. Thus your new time, uncorrected for reciprocity failure, is 2 seconds in this example.
Of course you should increase the exposure even more to correct for reciprocity failure.
If you’re using T-Max 100, the 2-second reciprocity correction is approximately 1/3 stop.
See the table on page 5 here:
You can consult the film maker's data for the approximate reciprocity correction required for the film you use.
As others have commented, the amount of time to add to compensate for reciprocity can only be determined acurately by testing with each exposure time used referenced to the meter readings.
Coming late but let me add my 0.02..
zero image cameras are fitted with an exposure dial as an accessory which helps you determine quickly the exposure needed at f/138 while you meter the scene say at f/16 with your lightmeter .. simply adjust the dial with your meter's f-stop and exposure value at ISO 100 to get the equivalent exposure at f/138 - really handy..
plus Fuji Arcos 100 behaves as a wonderful film having very little reciprocity failure..
I've found the Zero Image calculator to be pretty reliable, but anything over a second I double it, and anything over 10 seconds or so the calculator that came with my 4 by 5 Zero Image says to multiply it by 10 ... yikes! With ASA 100 my standard daylight exposure is 2 seconds, with kentucky windage for shadows, color of subject, my mood and so on. They all come out.
With the 4 by 5 I'm discovering that light fall off at the 25 mm setting is so severe I need to double it again so the edges don't go completely thin on me. I also have a Zero Image 2000 and their 135 (cute little bugger!) where this is not so much of a problem.
But as everyone else here said, experiment. When in doubt, add a few seconds -- I've found that pinhole seems much more forgiving of over exposure than a regular camera, probably because of the reciprocity stuff.
thomas, I have the same exact camera you have! been tempted by it recently. going to have to play. here i designed the world pinhole day logo last year and I haven't experimented yet myself so .... hope to post something soon and take part in the "big day" this year LOL