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  1. #11
    Vaughn's Avatar
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    Quote Originally Posted by Kirk Keyes View Post
    I'm dubious of "reaction time". It should be a very fast chemical reaction.
    I read up on it a little...In Christopher James' 2nd edition of Alt. Photo. Processes (p162), Mike Ware suggests that by slowing down the exposure, the print-out is allowed to progress to a greater degree than with quick hard exposures -- thus increasing the the self-masking.

    So it is not so much the "reaction time", as the non-instanteous creation of the print-out image.

    A more intense print-out image would help to retain shadow detail while the highlights progress. If this is what is happening, then it might help with other processes that have self-masking, such as salt prints and platinum/palladium prints.

    Vaughn

    A test of this theory could be done by giving a cyanotype a short hard exposure, check out the print-out image, then put the contact frame in the dark. Five minutes later, see if the print-out image has increased while it was in the dark.
    At least with LF landscape, a bad day of photography can still be a good day of exercise.

  2. #12
    David A. Goldfarb's Avatar
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    The self-masking property does make this all the more confusing. Say that one method causes the shadows to come in faster than the highlights--then the masking property takes over, and the highlights will continue faster than the shadows. This happens in both direct and indirect light, but maybe the rates are different. It's very hard to say what one is comparing at that point, because you can usually just wait until the highlights are where you want them for the contrast you're after, with albumen at least.

    Some have speculated that it has something to do with the Callier effect, but I doubt this, because I'm not convinced that the Callier effect is that important with a contact print, and because if it were the Callier effect, there would be more contrast with direct light than with diffuse light, and it's in fact the other way around.
    flickr--http://www.flickr.com/photos/davidagoldfarb/
    Photography (not as up to date as the flickr site)--http://www.davidagoldfarb.com/photo
    Academic (Slavic and Comparative Literature)--http://www.davidagoldfarb.com

  3. #13
    Vaughn's Avatar
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    David -- it may not a matter of the shadows coming in "faster" with one method over the other, but instead the darking of the paper due to the exposure (the print-out) lagging slightly behind the actual chemical change created by the striking of the chemicals by the UV light. Chemical reactions are not instantaneous -- they happen at different rates (tho some happen so fast that they appear to be instantaneous to us slow minded folks.LOL!) So I can picture electrons of the iron compounds being knocked off as the UV hits them...and this happening at a constant rate creating the exposure...but then it taking a little more time for the electrons to settle down and the chemicals to order themselves into new compounds.

    Well, I am probably just blowing smoke out my nether regions, but it is a fun metal exercise.

    Vaughn
    At least with LF landscape, a bad day of photography can still be a good day of exercise.

  4. #14
    Gustavo_Castilla's Avatar
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    My 2 cents !
    When using the sun you will get a higher contrast the down side is that it is rather difficult to get the same results a few days latter as exposure time will change , one of the more important things to consider is the radial heat ( how much heat your frame absorbs ) as this will change the exposure time and the ability of the chemistry to register gradation and contrast . and it is exponential buy the power of two .
    this means that as the frame gets hotter it drys the emulsion and it takes the power of 2 to generate an image .
    for example :
    first exposure bright sun mid day = 5 min exposure ( 75degrees at glass)
    second same conditions but glass is 95 degrees exposure will be 10 but it wont match the first one as the radiant heat and radiation affects the chemistry on the paper
    Gustavo Castilla
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  5. #15
    reellis67's Avatar
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    Quote Originally Posted by Gustavo_Castilla View Post
    My 2 cents !
    When using the sun you will get a higher contrast the down side is that it is rather difficult to get the same results a few days latter as exposure time will change , one of the more important things to consider is the radial heat ( how much heat your frame absorbs ) as this will change the exposure time and the ability of the chemistry to register gradation and contrast . and it is exponential buy the power of two .
    this means that as the frame gets hotter it drys the emulsion and it takes the power of 2 to generate an image .
    for example :
    first exposure bright sun mid day = 5 min exposure ( 75degrees at glass)
    second same conditions but glass is 95 degrees exposure will be 10 but it wont match the first one as the radiant heat and radiation affects the chemistry on the paper
    I only have the sun for printing historic processes and I can say that these are very valid points. The results I get are always different, which is both good and bad. Sometimes I get a nice surprise and sometimes I have to stop for the day because I just can't get a good print.

    - Randy

  6. #16

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    As an aside on the reaction times for the chemical reactions for formation of the image. Whilst the reactions themselves are fast, they are not occurring in solution where the photoelectrons produced can move freely to react further, reducing metal ions to image metal. The sensitiser is adsorbed onto the paper fibres, so diffusion of reaction products will become limiting in the overall image forming sequence - as Vaughn says, the print-out lags behind the actual photochemical reaction.
    Steve

    "You don't need eyes to see, you need vision" - Maxi Jazz

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  7. #17
    Vaughn's Avatar
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    FYI...

    I was up all night making platinum/palladium prints. My exposure times were very short -- 2 minutes to 9 minutes, depending on the neg. (on COT320, warm Pot oxalate)

    Just for fun, I gave the neg & paper half its exposure time, rested the paper for 15 minutes, then continued with the rest of the exposure. I suppose I should have done it both ways to see the tonal difference between printing straight thru and resting the print halfway thru, but I only had so much time and so much paper and chemistry.

    But what I did notice was the amount of POP image (the border around the negative) when I took the printing frame from under the lights halfway thru exposure -- and visually compared it to what I had 15 minutes later (I just turned the printing frame face down on the table.) There was a very significant increase in the amount of POP image after the 15 minute rest. So, depending on the amount of self-masking the POP images does, allowing enough time for the POP to print itself should have some affect on the tonality of the print. The prints did look nice...

    Vaughn
    At least with LF landscape, a bad day of photography can still be a good day of exercise.

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