Measured Reciprocity Failure of Ilford Delta 100

Discussion in 'B&W: Film, Paper, Chemistry' started by andrew.roos, Dec 19, 2012.

  1. andrew.roos

    andrew.roos Member

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    I recently purchased a Gossen LunaPro SBC light meter and was given the LAB darkroom attachment by a forum member. I used this to measure the reciprocity failure of Delta 100 in 120 format. I exposed a roll from a uniformly illuminated white subject (my enlarging easel illuminated only by the enlarger with no colour filtration) with shutter speeds in 1-stop intervals from 1/2 second to 512 seconds (8 minutes 32 seconds). The exposures were chosen to give roughly Zone V (middle grey) at ISO 100 in the absence of any reciprocity failure. I then measured the densities of the resulting negatives using the LunaPro LAB attachment. The additional exposure (reciprocity failure correction) required to obtain a Zone V exposure was then calculated from the HD curve in the Delta 100 datasheet, and plotted on a logarithmic axes.

    Delta 100 Reciprocity Failure.JPG

    I found that no reciprocity failure correction is required for measured exposures of up to 16 seconds (by "measured exposure" I mean the exposure indicated by a light meter). For measured exposures longer than 16 seconds, and additional 1/3 stop of exposure is required for every stop (doubling of exposure time) by which the measured exposure exceeds 16 seconds. So for example if the measured exposure is 30 seconds, which is approx. 1 stop more exposure than 16 seconds, then an additional 1/3 stop would be required to correct for reciprocity failure, giving an actual exposure of about 38 seconds.

    Note that this is to correct the Zone V densities. Since negative film has much more latitude for over-exposure than for under-exposure, it makes more sense to me to correct the Zone III densities where shadow detail is typically placed, in order to retain texture, and allow over-exposure of the highlights rather than under-exposure of the shadows (this can be corrected during development by reducing the development time, or in printing by using a softer paper grade). To do this, simply change the heuristic to say no additional exposure is required for 4 seconds, and then add 1/3 stop additional exposure for each stop of exposure beyond 4 seconds. In this case a 30 second exposure is 3 stops longer than a 4-second exposure, so requires an additional 1 stop of exposure, resulting in a 60 second actual exposure.

    These characteristics are quite different from the curve published in the Delta 100 data sheet which suggests a 155 second adjusted exposure for a measured exposure of 30 seconds.
     
  2. L Gebhardt

    L Gebhardt Subscriber

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    I think you should really use Zone I as the exposure to gauge reciprocity failure since it's where you will first loose detail. So based on your numbers it should be exposures greater than 1 second. Then add 1 2/3 of a stop to the exposure. This will also get you closer to Ilford's number.
     
  3. michael_r

    michael_r Subscriber

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  4. andrew.roos

    andrew.roos Member

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    Thanks for your comment.

    My logic was that I normally aim to place shadows where shadow detail is important not lower than Zone III. By targeting the Zone III density, the density difference between II and III will increase as the density of II will be reduced by reciprocity failure while the density of III will remain the same, so my shadow detail will be preserved. However there will be some compression between zones I and II, which I can live with in return for not making the highlights too dense (since they suffer less reciprocity failure); but of course this is a personal decision and your preference is equally valid.

    I make 1 2/3 stops above 30 seconds to be about 95 seconds so although using Zone I as the reference brings the result closer to Ilford's recommendation of 155s it still falls well short.
     
  5. andrew.roos

    andrew.roos Member

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    Thanks for the pointer to this very interesting article.

    I see he also targets Zone III and his curve lies roughly between my Zone III and Zone V curves although he gets a somewhat smaller correction (around 0.25 stops per stop of measured exposure).
     
  6. BradS

    BradS Subscriber

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    Andrew,
    Very interesting work. Thanks for sharing your methods and findings. I use Delta 100 for pinhole photography. Your findings agree well with my own "gut feel" and many iterations of trial and error.
     
  7. andrew.roos

    andrew.roos Member

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    Thanks for the confirmation, Brad.

    Andrew