Cyan-filter goes ND !?

Discussion in 'Enlarging' started by henpe, Feb 6, 2012.

  1. henpe

    henpe Subscriber

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    I have a LPL C7700MX / Omega 670MXL Dichroic enlarger (with colorhead). I use this solely for B&W-work and consider doing the following modification:
    I intend to replace the cyan filter with an equally sized steel-plate on to which I have drilled a number of small holes. The total area of the holes is 1/8th of the area of the “non-drilled plate”. When I dial in “maximum cyan”, the plate covers the light-path completely and only 1/8th of the light pass through, which should equals stopping down 3 stops. Hence, I have made myself a ND-filter, continuously adjustable from 0 to 3 stops.

    Is the idea presented above sound? Should I go for a few rather large holes, or many small holes? Are there any pitfalls that I have not considered (diffraction perhaps?)? Any other suggestions to implement a neutral density filter into my colorhead?

    For reference, I have attached an picture depicting the filters in place (the cyan filter goes first in the light-path, i.e. is located most backwards in the filter-stack):
    filters.jpg
     
  2. ic-racer

    ic-racer Member

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    Yes, a non-diffracting aperture is a good idea if you have printing times too short. I don't think I have ever had that unique opportunity :smile:. Omega incorporate a similar plate whith holes into the Chromega dichroic heads. Durst use a variable 'barn-door' type of shutter between the lamp and the filters.
     
  3. bdial

    bdial Subscriber

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    Like ic-racer, too much light has never been a problem with my color enlarger. The head has a built-in white attenuator, but I've never needed to use it.

    OTH, cyan filtration along with yellow and magenta gives you ND anyway, though it would likely take some experimentation to figure out how adding some cyan affects the contrast. You would also need to use the "two-filter" Y/M filtering method. A meter would be helpful too, though not essential.
     
  4. ic-racer

    ic-racer Member

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    True, but the OP indicated B&W printing, with presumably orthochromatic medium. In that case the cyan does not affect the paper (it is anti-red).
     
  5. henpe

    henpe Subscriber

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    I have seen threads here on APUG suggesting that cyan filters are not as "steep" and ideal as one might think, which can result in that also blue and green wave-lengths are filtered. This, in turn, could result in that the contrast of a VC-paper will be varied, slightly, upon adjusting the cyan filtration. This "fact" was the root to my idea of replacing the cyan filter.
     
  6. Doremus Scudder

    Doremus Scudder Member

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    Equal amounts of yellow and magenta equal neutral density as far as the paper is concerned. (Basically a graduated red,filtering both blue and green. ND filters of optical quality mounted on the lens is another possibility (most of us have one or two rolling around somewhere.

    Sure, the proposed solution will work, but seems a lot of trouble to go to when there is built-in ND capability in the form of yellow/magenta filtration already.

    Best,

    Doremus

    www.DoremusScudder.com