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  1. #1

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    Filtering equivalence for in-camera RA4

    This is sort of tangentially related to the RA4 reversal processing thread, but not so much that I'd get away with mentioning it there ...

    Anyway, I want to shoot RA4 in-camera as a paper negative. Probably Kodak Supra Endura. Having read around and talked to a couple of people who have done this, there's a bunch of difference approaches. The idea I guess is to get the colour balance on the negative as accurate as possible (I actually want to reversal process them eventually aswell, hence the tangential connection with the above).

    So in order to do this we need to ...
    • Filter for the magenta mask on the negative base.
    • Filter for the fact that RA4 paper is tungsten sensitive
    • Add filtration corresponding to the starter filter pack recommended for the paper we're using.


    And typically then mess around some more with some small filtering adjustments to get the colour balance spot on.

    I've seen this done seperately ... ie 85B for the daylight/tungsten conversion if shooting in daylight, then some unexposed but developed sheet film to filter for the negative mask, and then some CC or CP filters to make up the starter filter pack.

    I've also seen it done just using filter packs , ie that photo.net thread, where for example in one shot he uses
    155Y + 75M (and some UV + IR filters).

    So, the question is, is there some way of finding out the equivalence here between various named or un-named filters and their CMY equivalents. Is there a table somewhere that gives, for example, all the Wratten filter numbers and the filtration in CMY form that you could use instead ? IE an 85B corresponds to 20Y + 10M or whatever. Is there some way of working out what the CMY equivalent of the magenta mask on C-41 film is ? (or at least some reasonable average of mask colour, I know most C-41 films have differently coloured masks).

  2. #2
    Photo Engineer's Avatar
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    Color negative paper is basically tungsten balanced. See the in-camera work and filter packs used by Bujor B. on Photo Net.

    PE

  3. #3

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    Quote Originally Posted by Photo Engineer View Post
    Color negative paper is basically tungsten balanced. See the in-camera work and filter packs used by Bujor B. on Photo Net.

    PE
    That's the thread I've referenced above, I've pored over that thread several times. I'm trying to break that down though into it's component parts. He uses/used a sort of monolithic filtering approach. I want to know what portion of that filtering is the 85B, what part down to the filtering neccessary for the orange base of the negative, and what is straight colour balancing.
    In a general sense I was also curious if there is a definitive list relating those common filters to their corresponding CMY filtration values.

  4. #4
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    Without a color densitometer, I could not relate camera filters exactly to C/M/Y filtration. I would start with the common filter used with tungsten films in daylight and then add a strong UV filter. That should get you close. Use about an ISO 12 - 25 with the paper for starters. At that point, start adding CC filters to trim the color balance.

    PE

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    Ok, that sounds like a good start. How about compensating for the orange mask on the negative ? I've read a previous post of yours in which you explain that it's not a mask as such, but a positive orange image formed where the dyes aren't developed (with apologies for the transliteration, I understand the explanation I think) , so I'd guess that you'd have to compensate for this to get good whites, right ?

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    You do not need to compensate for the missing mask. You just get color contamination which would not be present with the masked negative. Colors will be degraded.

    PE

  7. #7

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    Finally got around to actually shooting a few sheets on sunday. This is one of the results. This is basically just inverted and levelled. No relative channel adjustments made.



    This was filtered with an 85b and the recommended starting filtration on the pack of Supra Endura that I got, 65m and 55y.
    As you can see it's still heavily blue. So do I add more magenta & yellow ? Or less ? I'm guessing more, but just on the offchance that I have it backward.

    In addition there's obviously a heavy degree of IR sensitivity on the paper. So I guess I need an IR cutoff filter or something similar ? It makes colour balancing the resultant scan pretty much impossible. Here's a more processed version of the above ...



    The IR does add a sort of glow to the portrait though

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    No there's no IR sensitivity at all, but as PE said earlier, it's very sensitive to UV.

    Nice results!

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    Quote Originally Posted by hrst View Post
    No there's no IR sensitivity at all, but as PE said earlier, it's very sensitive to UV.
    I had a layer of UV filtration mixed in with my filter pack. The reason why I say it's sensitive to IR was looking at the red channel of both images, the one of my sister isn't so bad, but this is the one of my dad:


    I've never seen such a pronounced effect shooting B&W film with (say) a 25a, or in the red channel of a colour negative. I've seen similar though shooting people with SFX with its near IR filter in place. It seems very pronounced.

  10. #10
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    In RA-4 paper, the red-sensitive layer is the topmost layer. So, the layers are in reverse order compared to film. And there is no UV filter layer. UV exposes the red layer.

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