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Calibration of Paper Negative Exposure Index

  1. Joe VanCleave
    I know that there are those who relish laboratory-accurate calibration procedures, complete with charts and graphs and 8x10 glossies (to quote Arlo Guthrey); however, I'm more of an empericist when it comes to testing.

    I like to do darkroom developing runs of negatives shot in the kind of light I'll be expecting to encounter, using the kind of lens and/or camera I'll actually be using, and emperically find the best Exposure Index, using the materials and methods at my disposal.

    That said, for exposing Arista's grade 2 RC paper (my standard paper negative of choice) in pinhole cameras, and developed in dilute Agfa Neutol WA or Ilford Universal Paper Developer, I've found it helpful to use an Exposure Index of around "3". In actual use, I will meter a scene using EI=3, find the exposure time that's adjacent to an F-stop that's 1/10 of my pinhole camera's f-ratio, then simply multiply the resulting time by 10 to arrive at the recommended exposure time. Example: At EI = 3, in EV13 light, the meter recommends F/32 @1.5 seconds; since my pinhole camera is around F/320, I'll simply multiply the 1.5 seconds times 10, yielding a 15 second recommended exposure time. (These numbers are hypothetical, BTW.)

    This method works for me because my handheld Gossen Luna Pro F's F/stop scale only goes up to F/128, whereas most of my pinhole cameras are over F/300.

    Before using this method, I simply ran calibration tests in each kind of light I expected to be shooting in, noting the EV light value and the resulting best exposure time for each pinhole camera I used. This was where, after many such tests, I arrived at an Exposure Index of around "3" for my paper.

    Well, that's all well and good for pinhole cameras. But I had also read of other's experience, via the I-nets, where they found a personal EI much higher than "3", in some cases as high as "25". I wanted to find out for myself, using a glass-lens camera with known accurate shutter.

    As a background, along with pinhole cameras, I've been exposing paper negatives in a Speed Graphic, using the curtain shutter for timing the exposure. I've used the Kodak Ektar 127mm lens, and also a simple 150mm binocular lens; I've also exposed paper negatives in a homebuilt foamcore nested box camera, using either the binocular lens, or even a plastic credit-card sized fresnel magnifier as a lens. In all of these cases, I've rated the paper at an EI of "3", and have had mostly good luck. In the case of the foamcore box camera, the exposures were all timed by hand using a lens cap as a shutter.

    Well, last week I decided to roll some grade 2 paper into a discarded B/W 120film paper strip and load it into my Bronica ETRS. The results can be seen here, and here.

    The upshot is that, for these tests, I discovered that using an EI of "3" would grossly over-expose the paper; I ended up settling on an EI of between 16 and 25. I then verified these results that same day using the Speed Graphic and curtain shutter, and they remained essentially the same.

    This shocked me, since I've been using the same paper in the same camera, with the same curtain shutter and lens, at a much lower EI.

    So this week I tried doing more exposures in my yard, rating the paper at EI = "25", and now the results look like the paper is once again much slower than before, closer to "3" once more.

    So why is my process changing all over the map? Well, there may be several causes, but I think it all boils down to the developer.

    One of the strategies I've used over the years to control excess contrast has been a dilute developer, and extending the developing time, in order to catch the negative when it's just right. In the process of doing this I often keep an old solution of used developer and just add some fresh replenishment of concentrate; this, too, has helped make the developer slow-acting as per my requirements.

    But, I often do not carefully monitor developer temperature; I'm aware of when it's really cold (like overnight in the cold garage-based darkroom) and will microwave it to warm up, but I really don't monitor the temperature with a thermometer.

    So what happened last week with the results that showed an EI of "25" is that I poured up a fresh mixture of Ilford developer 1:13, using water a bit warmer than recommended; I don't know what the temperature was, but it felt "tepid", so it may have been 80-90 f. And this week, when the results were again much slower, it was the same batch, after having sat covered for 2 days, and was much cooler in temperature.

    So now this leads to the next step of tests, which is to do a series of Exposure Index tests, using only fresh mixtures that are 80-90f in temperature. I will be doing this not only in my Bronica, but more importantly in my Speed Graphic and pinhole box cameras.

    I suspect there is some reciprocity failure with paper that I haven't yet identified; with pinhole cameras the exposure curve seems relatively linear, as with every full EV drop in light intensity I can double the daylight exposure time and get good negatives, when rated at EI = "3"; this is the reason why I've stated in other posts that paper negatives seem relatively linear and free from reciprocity failure. With glass lenses and sub-1 second exposure times I may find out differently.

    There's also the matter that the curtain shutter of the Speed Graphic is WWII in age, and probably never has been calibrated; I should at least take it down to a camera shop and have them test every speed combination for its true working speed.

    So I'll keep this group, along with F295, updated on the results of my fresh warm chemical speed testing. It would be cool if I could get good results, with acceptable contrast, at an EI of 25.

  2. Jerevan
    I just happened to notice this; a bit late, eh? But nevermind, it is interesting observations you've done. Please report back on the speed testing. Temperature may play a vital part as you say. I wonder what would happen if you ran the negatives through a Jobo or other temperature-controlled machine? So much fun to discover, so little time...
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