Pinhole Exposure Formula

Discussion in 'Pinhole Photography' started by Malco_123, Mar 28, 2009.

  1. Malco_123

    Malco_123 Member

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    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.
     
  2. Lee L

    Lee L Member

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    Try Pinhole Designer, which runs on MS Windows and under Wine with linux.

    http://www.pinhole.cz/en/pinholedesigner/

    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.

    Lee
     
  3. Malco_123

    Malco_123 Member

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    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).
     
  4. Christopher Walrath

    Christopher Walrath Subscriber

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    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.
     
  5. 2F/2F

    2F/2F Member

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    Hi,

    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 a moderator: Mar 28, 2009
  6. Lee L

    Lee L Member

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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.

    See the recent thread on reciprocity for sheet films:
    http://www.apug.org/forums/forum37/60348-comparative-reciprocity-failure-available-sheet-films.html for some of the issues and problems involved, and for some good relatively recent data for 5 films.

    Lee
     
  7. Pinholemaster

    Pinholemaster Member

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    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-Extended-Range-Exposure/dp/B00009R8J0
     
  8. Lee L

    Lee L Member

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    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/articles/200705/0403Bond_Reciprocity2.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.

    Lee
     
  9. DWThomas

    DWThomas Subscriber

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    That was my experience. I used PHD with Delta 100 in 2007 and felt that it overcompensated.

    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. :tongue: )

    DaveT
     
  10. Lee L

    Lee L Member

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    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.

    Lee
     
  11. Lee L

    Lee L Member

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    Here's a sample of how much sources can vary on recommended reciprocity adjustment. The attached table shows a number of sources for information on Ilford Delta 100 that Dave tried.

    The first column is the metered exposure in seconds. The second column is data from tests by Howard Bond in 2003, with data within about 1/3 stop, tested at each setting. The Covington/Schwarzschild adjustments from 1996 are based on negative density with a 10 stop ND filter at 128 seconds compared to a 1/8 second (0.125 seconds) exposure density with no filter, and is about +/- 1/3 stop in accuracy.

    The Curvexpert column is a smoothed best fit to the Bond data.

    The photo.net posting is a very close match to the Ilford generic suggestions for B&W films, and derived from their graph.

    Bond's data is the most thoroughly tested, and very likely the most accurate you'll find. The Covington adjustments are from a single real world observation, but match the Bond adjustments reasonably well. The other suggested adjustments are apparently pretty far off the mark from actual testing.

    As Bond suggests in his article, one gets the feeling that there's a lot of very outdated generic reciprocity (mis)information floating around.

    Pinhole Designer is about 3 stops greater than Bond's tested adjustment from 240 seconds, over 2 stops greater than Bond's tested adjustment from 120 seconds, and always more than a stop in excess of Bond's adjustment times.

    Lee

    I added a graph to show the information comparison at a glance. Looks like it got autoshrunk and jpegged, so it's not as readable as intended.
     

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    Last edited by a moderator: Mar 29, 2009
  12. DWThomas

    DWThomas Subscriber

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    Thanks Lee, looks like some good data to chew over a bit. I haven't decided exactly what I'm going to do for this year's WPPD, but if I go with the "SQ-Hole" I will probably try Acros 100, claimed to be relatively low in reciprocity effects. My other option is to use my 4x5 and clean up some aging sheet film here.

    In general I do prefer to optimize my exposures, but when working with pinholes, I sometimes have a flash of revelation. Most B&W has a fair amount of latitude, and ignoring reciprocity (which softens the effect) going from 30 seconds to 2 minutes is only 2 stops! It almost appears once you're in the ballpark, you can't help coming up with something usable. Last year I shot some Portra NC in the SQ-Hole, taking shots in pairs, one stop apart. When I got the film back from commercial processing, I really had to study the prints to see any difference.

    DaveT
     
  13. Lee L

    Lee L Member

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    Very true Dave. Pinhole shooters are more likely to be comfortable with "seat of the pants" operation and some slop in exposure, and a 1 stop overexposure certainly isn't deadly. Two stops over you can cope with some of the time.

    But being three stops overexposed using "standard recommended" adjustments could be very frustrating and disappointing for a first-time pinhole shooter, who might assume it's their own failure, and may not have bracketed far enough to get a good exposure. Three stops overexposure is also enough to make a negative very difficult to deal with.

    You're right about Acros. See the sheet film reciprocity thread that I referenced earlier in this thread for some more info on Acros.

    Lee
     
  14. Tom Miller

    Tom Miller Member

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    A simple approach, and the one I've used successfully for many years, is to test each camera and film (or paper) combination on a bright, sunny day to see what gives the best exposure. The exposure time that gives the best exposure is the camera's "magic number."

    I always use incident metering, and on my meter, a bright sunny day any time of year is EV 15.5 when the meter is set at ISO 100. EV 15.5 at ISO 100 is a constant. Let's say the camera/film's magic number is 5 seconds in bright sunlight.

    When photographing any scene, take an incident meter reading at ISO 100 and adjust the exposure accordingly. Bright sun doesn't need an exposure adjustment. Haze in the sky may need a one-stop adjustment - a ten second exposure. In shadow or heavy overcast, if the incident reading is EV 12.5, you'll need to expose three more stops (15.5 - 12.5) or eight times more than five seconds; a 40 second exposure will give you the results you want.

    I've used this method for up to seven stops more exposure than bright sun and achieved good results. I think this is because the initial test using a pinhole aperture gives such a long exposure that there is no need to compensate for reciprocity failure; the caibration test for the paper/film was done in conditions where reciprocity failure happens.

    Every paper or film has its own unique reciprocity characteristics. There is no formula that covers them all. Test for good results.
     
  15. zydeholic

    zydeholic Member

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    You might want to check out a Black Cat exposure guide. Cardboard thingy with a rotating dial on it that you can use to dial in new calcs.
     
  16. zydeholic

    zydeholic Member

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    I just downloaded this program, and plugged in Superia 100 and Superia 400, and got the same exposure times. I plugged in Superia 800, and got exposure times twice as long. Am I missing something?
     
  17. zydeholic

    zydeholic Member

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    "This Program" being PinholeDesigner.
     
  18. zydeholic

    zydeholic Member

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    Looks like reciprocity failure is the culprit.