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  1. #1

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    Cure for light fall off

    Hi folks, I have recently cobbled together an 8x10 enlarger with an Arista w31 10x12 cold light. I use a roscoe cc40y to enable vc printing (still not spot on with the contrast grades, but I can work with it). The problem I am having is light fall off at the corners. 1 to 1 2/3rds stop difference from center to corners. It's not my lens (240 rodagon) because even without the lens I get the same readings. It is a major pain doing mural sizes (40x50 inch horizontal) because the exposures are really long (5 or 6 minutes) and then I have to burn in the corners another 7 or 8 minutes to compensate for the fall off. Is there such a thing as an inexpensive center filter I could attach to even out the exposure. I tried to make an 8x10 blank print and develop it and then scan the print and try to make a negative on ohp transparency stock and use it as a filter but I'm not having good luck with that. What I was hoping is that there is some kind of center filter I could attach to the lens and be done with it. any thought out there? i would appreciate the help.
    regards
    Erik

  2. #2
    Christopher Walrath's Avatar
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    Wirelessly posted (BlackBerry9000/4.6.0.167 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102 UP.Link/6.3.0.0.0)

    Just a quicky idea. Cokin makes center spot filters. Might be able to make something work.
    Thank you.
    CWalrath
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    "Wubba, wubba, wubba. Bing, bang, bong. Yuck, yuck, yuck and a fiddle-dee-dee." - The Yeti

  3. #3

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    Thanks Christopher, I need something like that but in reverse - I need density in the center of the filter and clear towards the edges. Maybe there is hope with an inexpensive filter. Those filters were only about 15 bucks

    regards
    Erik

  4. #4

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    Erik,

    I've had an idea that might work, if you use a piece of dupe film the same size as your light source, expose it to get a medium to light gray and then after processing, tape it to your light source (sort of how plume uses a scrim to correct for light falloff.


    erie

  5. #5

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    I agree with epatsellis, it should work.
    In that way you hold the center part back in exposure in the same ratio as the light-fall-off you have at the corners right now.

    Peter

  6. #6

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    Thanks guys, that's kind of what I did when I made a blank grey print and made an ohp transparency with my inkjet and put it above the negative. I guess my skills at making a decent ohp are not very good. In theory it should be the best way to go but I haven't had good luck so far. Thanks for the suggestions.
    regards
    Erik

  7. #7
    ic-racer's Avatar
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  8. #8

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    Ic-racer, thanks! I read your posts and that is where I got the idea to try and make my own filter on ohp. I just wasn't very successful with my ohp printing. How many stops did you lose because of the added density with your filter set up? I appreciate the help.
    regards
    Erik

  9. #9
    ic-racer's Avatar
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    Quote Originally Posted by Erik L View Post
    How many stops did you lose because of the added density with your filter set up? I appreciate the help.
    regards
    Erik

    Actually none at the edges

    I suspect it is about a stop at the center.

    I had read about people who use gray paint on the lamp housing, behind the central portion of the light grid. With any of the methods, I don't think there is any getting round a lot of trial-and-error to get it right.

  10. #10
    RJS
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    The Arista 10X12 should cover 8X10 just fine. I wonder if the problem is not in the lamp house - you don't mention what the housing is. Age can cause discoloration and changes of various sorts; I would be looking there very carefully before trying to balance with a filter.

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