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  1. #1
    Usagi's Avatar
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    Exposure time for step wedge?

    Hi,

    I would like to use my step wedge (trasparent) for getting characteristic curves of the films easily.

    If I have figured out everything properly, then best way to expose step wedge to the negative is contact printing.

    That's okay so far... But how do I determine exposure time?

    If I do contact printing under the enlarger - where do I measure time? And in case of exposure meter, what aperture setting I should give to it?

    Or if I do contact printing at the dark room with white ceilings using flashes, how do I measure proper flash power for the print?

    So far I have used step wedge by putting it to the film holder, top of the film like a some kind of sandwich. Then I have exposed 'evenly' lighted surface. This way I get some results, but evenly lighted surface. It seems to be next to impossible.

    Any tips where to start?

  2. #2
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    I am just going through Phil Davis's BTZS. It says set the f-Stop of the lens to f11 and roughly in focus. Raise the enlarger head to the point where a incident meter set for ISO 100 reads E.V. 4. At this point the exposure time should be 0.4 for 100 speed film.
    D-76 is a standard developer, although not one I use.
    Ansel Adams - The Negative

  3. #3

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    Step wedge exposure

    I tape a stouffer step wedge to 5x7 film in the holder. I cut a piece of white translucent plex to fit in filter holder adapter for front of lens. I aim camera at a white wall, read exposure with meter through white plastic, this would give a zone 5 exposure, I increase 5 stops for zone 10. Slip plastic in front of lens for exposure, shoot exposure at the +5 stop.

    This would give a worst case situation for camera flare, but I use it to get one exposure for the full 10 stop range.

  4. #4
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    I run all my film tests by exposing the wedge, with the lens set at infinity for a completely out of focus image of a white or gray mat board as my target. The mat is placed always on the shady side of my house on a sunny day with a clear blue sky (you want no chance of a cloud, even a very thin cloud, moving in front of the sun as the EV will change in the shade). Or, on an overcast day with even cloud cover, as long as the EV remains the same.

  5. #5

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    Jukka, I recommend reading “Beyond the zone system” by Phil Davies. It is a good investment and like trexx writes it covers this aspect of film testing on page 70 and 71.

    Quote Originally Posted by trexx View Post
    I am just going through Phil Davis's BTZS. It says set the f-Stop of the lens to f11 and roughly in focus. Raise the enlarger head to the point where a incident meter set for ISO 100 reads E.V. 4. At this point the exposure time should be 0.4 for 100 speed film.
    I might add that the reading should be done in reflective mode if you point the meter towards the enlarger lens. Another way is to place a white card in the middle of the illuminated area made by the enlarger and measure the light with a spot meter.

    I find that Davies’ exposure recommendation is a good starting point, but I would only make one exposure and check if it is correct before exposing all the sheets required to do a film test.

  6. #6
    Usagi's Avatar
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    Thank you! The matboard would work, but now it is quite dark here in Finland.. So I will test Davis's method.

    I have BTZS, but have to say that I haven't looked it for couple of years... Embrassing :/

  7. #7
    Usagi's Avatar
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    After reading BTZS, thinking, etc... I ended to using Stevew's method. I must be really dumb, before that I always but stouffer with film sheet to the holder. I was really a pain in the ... Because there is not much room inside of the film holder.

    After reading Stevew's post I realized that I can tape the wedge outside of holder, so that film can be quickly changed

    Now I have done series of test, results are really promising.

    I could have used BTZS approach as well, but the final reason that pushed me towards to camera approach was the current situation with my darkroom. New darkroom is under work and currently only way to use enlarger is to carry it to the bathroom in couple of pieces and assemble it there.

  8. #8

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    There are a couple of things to remember about using contact printing methods with a step wedge. The first is reciprocity. Some films have significant reciprocity failure at exposure times as short as a tenth of a second; others hold their speed even at 10 seconds. Check the manufacturer's recommendations, and make appropriate adjustments. The second is that it takes a significant time for the enlarger lamp to warm up and to cool down. Light is produced during these times, but it is not as bright as the steady state values. Generally the lamp warms up to full brightness in about a tenth of a second, but it may produce significant light for one-half second after it is turned off. Using the enlarger timer for exposures of less than two seconds will not be anywhere near accurate, although results will be repeatable for any given exposure time setting. That means that the method works for producing curves and for relative measurements, but it will not give accurate film speeds. I rigged up an old 4X5 film pack adapter so that I could make a contact exposure in the camera. That means I can use the camera shutter to time the exposure, and I don't have to worry about this stuff.

    As for the exposure time, measure the light at the exposure plane, and give five or six stops more exposure than indicated. Most step tablets have a density range of about 3 (10 stops), and this will put middle gray at about the middle of the step tablet.

  9. #9
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    Well, it seems to me that you have missed the fact that if you are using an enlarger or doing it in-camera, the exposure and light temperature should be that for which the film is optimized.

    Therefore, using an enlarger, you should have a daylight filter set in the beam to make it less like tungsten! For me this is 100 C and 30 M. The exposure should be in the range for the film and if you cannot do this it should be in the range in which reciprocity does not rear its ugly head. I use 1/2" at f22 which is within the low to no reciprocity range of the film I use.

    Just some thoughts.

    PE

  10. #10

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    I use an 80A filter over the lens of my enlarger.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

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