uneven light (fall off) beseler enlarger produces cyan cast

Discussion in 'Darkroom Equipment' started by lhalcong, Mar 3, 2013.

  1. lhalcong

    lhalcong Member

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    While working on a different project, I decided to meter the light falling on the baseboard of my Beseler 23C-II XL Dichro Head , using my sekonik incident meter in EV mode. To my surprise, the measure in the center of the board (below the lens) is higher than on the corners, in fact, there is more fall off as you move further onto the edges of the board. The fall off is between 0.1 EV to 0.3 EV. This is the case regardless of the height of the head.
    - Is this the reason I am getting a partial and light cyan cast only in the center of the picture which only happens at longer exposures. (it doesn't at short exposures) ? Is this fall off normal , and if not, what to do to correct ?
     
  2. Fred Aspen

    Fred Aspen Member

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    While the light meter detects a slight fall off, it isn't really a problem. I see more than that with a couple of my enlargers but I don't detect any differences in my prints. A tenth of a stop is virtually nothing in the grand scheme of things. Have a beer and let it go!
     
  3. Chan Tran

    Chan Tran Member

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    Assuming the light source in your enlarger is perfectly even, still you should get light fall off because that's the nature of the lens.
     
  4. Prof_Pixel

    Prof_Pixel Member

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    Sounds like cosine fourth law of illumination falloff.
     
  5. Doremus Scudder

    Doremus Scudder Member

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    Learn to edge burn. There is not an enlarger lens made that doesn't exhibit some fall off.

    The cyan cast, however, is a bit of a problem... Check to make sure your dichroic filters are aligned correctly, check the lens coatings, check for internal reflections, etc.

    Good luck,

    Doremus

    www.DoremusScudder.com
     
  6. Mainecoonmaniac

    Mainecoonmaniac Subscriber

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    I don't own a 23CII, but a 45MXT. Is there w away to raise and lower the head between the negative stage? Your head might be too close to your negative. If you can raise the head away from the negative stage, you might get even coverage on your negative. Prof Pix may be right also about cosine fourth law of illumination falloff. Try tilting your meter towards the lens and you might get a brighter reading. To me, it's not a big deal since I burn in light areas of prints anyway.
     
  7. DREW WILEY

    DREW WILEY Member

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    Falloff can indeed create a differential reciprocity effect from center to edges at long exposures. But your meter is probably not correctly measuring it (cosine). Are you printing color paper? Ideally, the diffuser in your colorhead must be matched (ground) the match the falloff on your lens itself with respect to the colorhead. Otherwise, you can try a longer lens, better diffusion, a faster lens, etc.