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  1. #11
    Lee L's Avatar
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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    As Bond suggests in his article, one gets the feeling that there's a lot of very outdated generic reciprocity (mis)information floating around.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.
    Attached Thumbnails Attached Thumbnails Delta100curves.jpg  
    Attached Files
    Last edited by Lee L; 03-29-2009 at 11:24 AM. Click to view previous post history.

  2. #12
    DWThomas's Avatar
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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

  3. #13
    Lee L's Avatar
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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

  4. #14

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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.

  5. #15
    zydeholic's Avatar
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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.

  6. #16
    zydeholic's Avatar
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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?

  7. #17
    zydeholic's Avatar
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    "This Program" being PinholeDesigner.

  8. #18
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    Looks like reciprocity failure is the culprit.

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