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  1. #1
    36cm2's Avatar
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    Why doesn't 2+2 = 4 in print exposure?

    Can someone please explain to me why you don't get the same print results when you expose photo paper continuously for 16 seconds as opposed to exposing it twice for 8 seconds? I haven't tested papers yet, but am looking to standardize my process and read about this somewhere in my preparations.

    Thanks,
    Leo
    "There is a time and place for all things, the difficulty is to use them only in their proper time and places." -- Robert Henri

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    Kevin Kehler's Avatar
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    The key is the light drop-off and the timer. First, you are assuming your timer is 100% accurate which it rarely is, especially if it is a mechanical (as opposed to an electrical) timer. So the timer might be counting wrong. Second, the lamp takes part of a second to warm up and it shuts off part of a second either early or late, depending on your enlarger. So you might be getting more or less light than your timer says you are getting. I know my set-up has 2+2+2+2= 9 seconds uninterrupted exposure.
    Once a photographer is convinced that the camera can lie and that, strictly speaking, the vast majority of photographs are "camera lies," inasmuch as they tell only part of a story or tell it in a distorted form, half the battle is won. Once he has conceded that photography is not a "naturalistic" medium of rendition and that striving for "naturalism" in a photograph is futile, he can turn his attention to using a camera to make more effective pictures.

    Andreas Feininger

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    ruilourosa's Avatar
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    the enlarger system takes some time to inicialize, so... even if it´s few, take the time to the lamp to get full power, the transformator to send current, and the clock to make the passage and you will get less time in divided exposures
    vive la resistance!

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    L Gebhardt's Avatar
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    Depends on your enlarger. I have a color head which uses a shutter and multiple short exposures do add up to one long one. My other enlarger, using the Ilford 500 head, also seems to add up to the same (based on test strips). I'm sure there is some difference, but it isn't visible. So you really need to test.

    With an older condenser system I used there was a noticeable difference, which I attributed to either the lamp warming up, or the timer (older, but digital).

  5. #5
    Worker 11811's Avatar
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    Pre-strike your lamp, let it warm up and stabilize color temperature then use a dowser to cut the light on and off. You can make a dowser out of a solenoid and some bicycle cable attached to a metal shutter. There may be pre-made assemblies you can buy, as well.
    Randy S.

    In girum imus nocte et consumimur igni.

    -----

    http://www.flickr.com/photos/randystankey/

  6. #6
    RalphLambrecht's Avatar
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    Quote Originally Posted by 36cm2 View Post
    Can someone please explain to me why you don't get the same print results when you expose photo paper continuously for 16 seconds as opposed to exposing it twice for 8 seconds? I haven't tested papers yet, but am looking to standardize my process and read about this somewhere in my preparations.

    Thanks,
    Leo
    Timers, yes, and the 'Intermittency Effect'.

    http://jbhphoto.com/articles/intermit/intermit1.htm
    http://books.google.de/books?id=MblH...Effect&f=false

    The one reason why I don't like the Fred Picker way of making test prints.
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  7. #7
    36cm2's Avatar
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    Quote Originally Posted by RalphLambrecht View Post
    Bingo. Thanks very much for the answers, guys. It's been very enlightening.
    "There is a time and place for all things, the difficulty is to use them only in their proper time and places." -- Robert Henri

  8. #8
    MattKing's Avatar
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    It's probably worth noting that while 2 + 2 isn't the same thing as 4, 22 + 22 is awfully close to 44.
    Matt

    “Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”

    Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2

  9. #9
    2F/2F's Avatar
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    If you were to use a shutter and - less importantly, but still a factor - a voltage regulator, 2+2 would be much, much closer to 4.

    I don't use a shutter (though I do use a voltage regulator for all my darkroom equipment now that I have the one that came with my big enlarger). This is why I don't do stepped test strips. It was clear to me my first time printing, having had a little bit of electronics knowledge from working in power plants, that slop in mechanical timers and lamps ramping up and ramping down made it so that the best increment on a test strip never matched a full print at that time. So, I get a feel for what times are common for well exposed negs at certain enlargements, adjust things on the enlarger based on how far away from well exposed I think my neg is, and do a test strip all at one time. Sometimes I will do the same thing three times at three different times or three different apertures if I am less sure of what to expect.

    I also try to use exposures that are in the 15 - 45 second range, to minimize the effect that variations in light output throughout the exposure time have on the final print. A lamp's ramping up and ramping down time are larger and larger percentages of the total exposure time the shorter exposures get. It also makes burns and dodges more repeatable, for the same reason. You can never do a burn or dodge exactly the same every time. There will always be some differences in the motion of your hands or other go-betweens, and in the exact timing of their placement. These differences have a significantly greater effect the shorter exposure times become.
    Last edited by 2F/2F; 07-21-2010 at 04:05 PM. Click to view previous post history.
    2F/2F

    "Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."

    - Rob Tyner (1944 - 1991)

  10. #10
    Dan Henderson's Avatar
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    just for fun one day I made a test strip in the usual way of uncovering sections of paper on one half of a sheet with the other half covered, i.e. 2, 2, 2, 2, 2. On the other side I gave the same amount of time to each to the corresponding section of paper through black cardboard with a strip cut in: 2, 4, 6, 8, 10. The difference was remarkably visible and showed me that is the best way to make test strips. (I described the process using even time increments for illustrative purposes only. In practice I use the f/stop timing method: 2, 2.8, 4, 5.6, 8, 11, 16, 22 and 32 seconds.)


    web site: Dan Henderson, Photographer.com

    blog: https://danhendersonphotographer.wordpress.com/

    I am not anti-digital. I am pro-film.

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