Pinhole Exposure Formula
I am trying to narrow down a bit of the guess work involved with pinhole photography, and was wondering if anyone knew of an exposure formula that is fairly accurate.
Ideally the formula would take into account the films ISO speed, f stop, the strength of the light source, and would give the optimal time in either seconds or minutes.
The end result of this would be to allow me to have a idea of how long an exposure would need to be, based on a light meters reading.
Try Pinhole Designer, which runs on MS Windows and under Wine with linux.
It will do what you want, and I think print out a table of adjustments to carry along.
It has reciprocity built in for a number of films, but I can't vouch for how accurate the adjustments are in practice. In any case you could take notes and make the necessary adjustments.
This looks good, but you still wouldn't happen to know the formula the program uses though?
I would like to know the formula so I can have a greater understanding over how the exposures work, (as part of the same project I will be doing a silver emulsion and would like to find the ISO by using this formula backwards).
If you are going to be attempting to guesstimate the reciprocity failure of your own emulsion, that can be tricky without extensive research on your part by logging different exposures at different development times. As to various manufacturer's emulsions, they all, for the most part, differ, one from another. And the manu's publish their own testing results as well. I once worked out a curve for TMY for close to exact times (if you can have an exact time in PP) but anymore I just guess close. I use Pinhole Photography as a diversion.
On a related thought, you might try www.pinholeday.org . This is the site for the events on April 26 but you might gleen further resources/contacts here.
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The most difficult part of nailing pinhole exposures is figuring for reciprocity failure during long exposures.
The rest is straightforward if you have an approximate f stop.
IMO, you should figure this through tests to find working shutter speeds at certain EVs, rather than precise exposures factoring in awesome reciprocity charts. Figure out how long an exposure you need to get a good exposure at a certain incident exposure value, then keep notes. It's easier than figuring exact reciprocity characteristics (though basically the same idea), and makes more sense practically for a pinhole camera.
For instance, say your incident meter reads EV 15 give or take: sunny 16 conditions. Say your film is ISO 125. You expose at about f/16 at '125 for sunny 16. Your f stop, let's say, is f/360. Count f stops from 16 to 360: 22, 32, 45, 64, 90, 128, 180, 256, 360 = nine stops from f/16. So, count down nine shutter speeds from '125: '60, '30, '15, '8, '4, '2, 1, 2, 4. Thus, the equivalent exposure for sunny 16 at f/360 is four seconds.
When you go to take your pix the first time, write down the EV your meter read, or the lighting conditions if you don't use a meter, and its recommended shutter speed for your f stop. However, bracket using longer exposures, and take note of which ones are which. Look at your developed negatives. Contact print them on filters 2 or 2.5, or grade 2 paper, to a density that makes the film edges zone 0 after the paper dries. Pick the one that gives you the amount of detail that you will generally want in the shadows. Look at your notes to see what the shutter speed was. In the future, that is your actual shutter speed for sunny 16. You can do this same thing every time you encounter a different EV, and gradually you can build a direct-reading chart of shutter speeds for each EV. You must make a new chart for each camera and each film, of course.
You can also use the same contact prints to determine good developing times for these compensated exposures.
Last edited by 2F/2F; 03-28-2009 at 06:56 PM. Click to view previous post history.
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It's not a single formula. It varies by film, and the accuracy of the data varies as well. You could print out the times for f:22 in Pinhole Designer and compare the metered times against the adjusted times. I've done a regression on Plus-X reciprocity failure from the Pinhole Designer data, and it looks a lot like a generic Kodak or Ilford adjustment.
Originally Posted by Malco_123
See the recent thread on reciprocity for sheet films:
http://www.apug.org/forums/forum37/6...eet-films.html for some of the issues and problems involved, and for some good relatively recent data for 5 films.
Here's the basic formula. Know the effective ISO of your film. Know the f/stop measurement of the pinhole. Use a light meter to measure the strength of your light source. Consult the reciprocity chart for your film / developer combination. Then test, test, test, and test to see if all this information actually works for you.
My favorite tool is a Black Cat Exposure tool to work in very small f/stops beyond what my meter can show. Saves time trying to do the mental calculation in the field. http://www.amazon.com/Black-Cat-Exte.../dp/B00009R8J0
I just did some checking last night on the Pinhole Designer numbers, and the following films are given the same reciprocity failure adjustment: Delta 100, Delta 400, Delta 3200, FP4+, HP5+, PanF+. TMX and TMY are given a different adjustment. Plus-X, Tri-X, and TXP are grouped with another identical adjustment.
This data is in fairly strong disagreement with the data Howard Bond derived experimentally in his article on 5 of these films: http://www.phototechmag.com/articles...ciprocity2.pdf
Pinhole Designer is also pretty far off the mark from tested adjustments given separately by Robert Reeves and Michael Covington in their respective books on astrophotography. (Search APUG for their names for a complete reference on the books.) Films within each "group" given identical adjustments in Pinhole Designer also show very different behavior from one another in the real world tests I've seen.
So the recommended reciprocity adjustments in Pinhole Designer are very rough, and mostly indicate greater increases in exposure than testing would suggest. I would use them only as a basis from which to adjust, mostly to shorter adjusted exposures.
That was my experience. I used PHD with Delta 100 in 2007 and felt that it overcompensated.
Originally Posted by Lee L
It's a neat little program, but last I tried it, lacked some current films. I wish it could import some data from a text file for configuring film type and the magic numbers. (Yeah, every now and again I lapse into fantasy. )
I keep wishing they'd allow the user to modify the expressions used to calculate reciprocity failure, as it's nice to have their f:22 to working stop converter involved in that process. But for now a spreadsheet with better reciprocity data will have to suffice.
Originally Posted by DWThomas