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  1. #1

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    Analog Unsharp Mask

    I've heard it mentioned in texts, but I can't find any literature or someone with experience. Can you guys help me build analog unsharp masks?

    -Zac

  2. #2
    markbarendt's Avatar
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    Welcome to APUG.

    Type "unsharp" into the APUG search box.
    Mark Barendt, Beaverton, OR

    "We do not see things the way they are. We see things the way we are." Anaïs Nin

  3. #3

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    This technique, and related methods of enhancing detail in prints, are covered on pages 108-114 of Advanced Amateur Astronomy by Gerald North. The ISBNs for this book are 0748602534 or 0748603247. I've never done this, but the basic idea is to make an under-exposed and out-of-focus copy of the original negative, on negative film, and sandwich the resulting positive with the original in printing.

  4. #4
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    I've tried it once very quickly just for fun a few years ago when I was making my first Ilfochrome prints. It's easy in theory and a bit more complex in practice, as you need to have a precise registration system and have to somehow add some distance between the original and the mask to make it out-of-focus.

    You can do OOF mask and sandwich it with the original without gap, or a sharp mask and have a gap in the printing stage, or combine both of the methods.

    If you print negs, the mask is positive, if you print positive (e.g. Ilfochrome), the mask is negative. So, in any case, you make a copy on a negative film. Fine-grained, slow, low-fog film is of course best for masks. It doesn't necessarily need to be underexposed, but usually it is "underdeveloped" or developed in a weak developer giving low contrast so that the effect is only subtle. By changing the exposure of the mask film, you can adjust whether you want to affect only the highlights (underexpose the mask) or the whole image (expose so that you get full tonal scale in the mask), or the shadows (overexpose the mask greatly). Oh, these are for positive printing (Ilfochrome), for negs they are reversed.

    I just manually aligned the 35mm contact copy with the 35mm slide I was printing, so the alignment was a bit off. I just taped them together. But it worked as a proof of the concept surprisingly well!

    It's more difficult to control how much out-of-focus the mask is. IIRC, I just contact copied it through the base to make the mask slightly OOF, then I sandwiched it with the film so that the film base again adds some distance. For the actual OOF effect, the diffusiveness of the light source and the aperture of the enlarger lens affect the result greatly (in both stages). And, if you increase the distance between the mask and original too much, you have to somehow compensate for the different size. You may need to do enlarged or reduced mask, depending whether is it before or after the original in the light path.

    The mask in my example was highlight only (underexposed), I should have exposed it a bit more to give more overall effect, and developed less to compensate.

    So, here are some ideas to consider!.
    Attached Thumbnails Attached Thumbnails USM.jpg  
    Last edited by hrst; 03-25-2011 at 11:34 PM. Click to view previous post history.

  5. #5

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    This subject is described in depth in "Way Beyond Monochrome 2"...

    If you go to Ralph's website, www.waybeyondmonochrome.com, under TOC, there is a pdf of that chapter.
    Develop, stop, fix.... wait.... where's my film?

  6. #6
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    It's also used in typical dye-transfer printing, traditionally speaking, and many books relating to it in this respect are available. Try DA Spencer's 'Practical Color Photography' to name one.
    If you are the big tree, we are the small axe

  7. #7

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    There's a pretty good beginner's step by step with explanations in Bruce Barnbaum's book.

  8. #8
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    ... or check out Lynn Radeka's site, he wrote the masking section in Ralph's Way Beyond Monochrome and Bruce "strongly recommends" Lynn and Inglis' registration system. I can attest to Lynn's since I took his workshop and bought his system.
    “I drank what?” - Socrates

  9. #9

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    Dear Zac,

    Another good reference: http://ctein.com/booksmpl.htm

    Neal Wydra

  10. #10
    holmburgers's Avatar
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    Basically, masking is this: by adding a positive to a negative (or vise versa) that is lighter (less dense, e.g. 20% of the other one), it serves to reduce the overall contrast of the dominant image. It's a way to affect contrast for printing mediums that have little contrast-changing abilities.

    Furthermore, if the mask is slightly out of focus (unsharp), it serves to accentuate lines and areas of changing tonality, thus giving the effect of increased sharpness.

    By adding a fuzzy positive to a sharp negative, the edges in the print get sharper.
    If you are the big tree, we are the small axe

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