PDA

View Full Version : is this accurate?



timeruinsbelief
01-18-2006, 07:58 PM
so does this calculation really work?

meter at f16 that time is "X"
divide your pinhole f stop by 16... that # is "Y"
square Y and multiply that by X

and i guess that will give you your exposure time. does that sound right to anybody?

ImageMaker
01-18-2006, 10:30 PM
That's exactly correct, not counting reciprocity failure.

timeruinsbelief
01-19-2006, 10:19 AM
how do you figure that in?

ImageMaker
01-19-2006, 12:57 PM
Essentially, after applying the calculation for the aperture conversion (as above), you then increase the exposure by some factor that is dependent on how the film responds to very low light levels. All films lose sensitivity below some threshold light level; with the common types like Plus-X and Tri-X, that loss becomes significant at metered exposures of 1 second or longer (some films, like a couple Polaroid types, require significant correction from 1/10, and a few, like Fuji Acros, require none out to remarkably long exposures, so you have to know what your particular film wants). Every manufacturer publishes charts, at least for "professional" films (consumer films, often not, because consumer cameras made these days aren't capable of long exposures), that tell how much correction is needed at what metered exposure, but with the great bulk of "conventional" films (Plus-X, Tri-X, most Ilford films, APX 100 and 400, Fomapan 100 and 400, and Efke 100, just to list a few) the correction needed is simply to triple the time instead of doubling for each additional stop past 1 second exposure. T-Max and Delta films, Acros 100, and Foma 200 use a different halide composition and grain shape and require less correction, frequently starting at longer exposures; Acros 100 is the "fastest" film you can buy for low light pinholing because it requires so little reciprocity correction.

The published charts are often complicated by the manufacturer's recommendation of reduced development when reciprocity corrections are applied; that reduced development reduces the film speed, as well, which requires still longer exposures (and still more correction). For pinhole purposes, however, altering development for reciprocity conditions not only seems unnecessary but, given the wide range of exposures likely on a roll of film, downright ill-advised. Develop the same way you would with a lens camera and "normal" exposures, and use a simple rule for reciprocity correction, and you'll get good exposures out to an hour or more. The rule I use is "3 for 2" starting from 1 second on all "conventional" grain films. I haven't done pinhole with "designer grain" films like T-Max or Acros, but for T-Max I'd expect to use 5 for 4 starting at 2 seconds with TMY, and 5 for 4 starting at 8 seconds with TMX; TMZ-P3200 is more like 3 for 2 starting at 4 seconds, if I'm reading Kodak's chart correction; Delta films, according to Ilford, require the same correction as their conventional grain stocks, but I'd want to test to be sure. Foma 200 appears, based on the charts, to be about the same as TMY, and Acros 100 is a rule unto itself, with no correction required for less than 100 seconds and only 1.5 stops at 1000 seconds metered.

So, for conventional film like Tri-X or FP4+, you'd use this conversion:




Calculated
Actual


1

1


2

3



4

9



8

27



16

81



If, like me, you regularly use a scientific calculator, you can also take the base 2 logarithm (obtained by taking the common log and dividing by the common log of 2) of the calculated time, and raise 3 to that power; it's the same caclulation, but much faster.