No doubt some of you digital printers have been gloating at the hoops Mike has been jumping through to get himself established for traditional darkroom printing, as well as the myriad comments from all and sundry about the efforts they have to go through to control their printing environments. Problems with stray light, problems with light proofing, problems with safelights. All to make those unruly photons be where folks want them and not where they don't. How nice that digital printing is not heir to the same problems, don't you think?
The truth about shmoosh
When was the last time you had your scanner cleaned? I'm not talking about the exterior surfaces like the platen on a flatbed scanner or the film carrier for a film scanner. I'm talking about the interior optics—the back side of the platen, the first surface mirrors inside the scanning head, the relay lens that's between those mirrors or between the mirror and the CCD array (see figure 1). If you've got a scanner that's more than a handful of years old, I can pretty much guarantee that stray photons are degrading your scans. And it's almost certainly crept up on you so gradually that you never even noticed.
Here's the thing. Even though all those internal optical surfaces are protected from direct exposure to the environment, the scanner isn't airtight. Dust creeps in, especially with a film scanner, which can't really be well sealed at all. In addition, the plastics, lubricants, and other synthetics inside the scanner outgas. You know that smudgy haze of plastic volatiles that appears inside your automobile windshield and can make it really, really hard to see when you're driving into the sun? Scientists call this "shmoosh." And, yeah, your scanner is like that. Or it will be. It just takes time.
Fig. 1. This is what the typical light path in a flatbed scanner and a film scanner looks like. The exact arrangement of mirrors and lenses will depend upon the scanner, but this gives you the idea. Shmoosh will show up on the underside of the glass platen, on the surfaces of mirrors, and on exposed lens surfaces.
What's the effect of that veil of deposits and dust that's coating mirrors and lenses? If it's prints you're scanning, it'll be something like figure 2. In mild cases, the veiling will be uniform and you may not even realize it's happened and that you're losing shadow detail. In severe cases, the deposits can be nonuniform, especially on the underside of the platen, and you see tonal variations in your scans that aren't in your originals but appear in the same place every time you do a scan.
In film scans, you may get the same kind of degradation as in figure 2, but the highly directional transmitted light and the high density range in some films (color slides and high-contrast black-and-white negatives) means you're also likely to see haloing, like in figure 3. When the highlights start bleeding, it's time for cleaning. Actually, it's way past time.
Fig. 3. Film scans can not only suffer a loss of contrast in dense areas but also have haloing when there is shmoosh in the light path. In positive scans, like what's shown here in the diagonal upper half, the halos will be light intruding into dark areas. In negative scans, it will be dark bleeding into highlights.
No help from me
Now here's the bad news. I won't tell you how to do this yourself. There are so many ways to go wrong and ruin your scanner. If you don't get it reassembled exactly the way it was when you took it apart, components may be out of alignment and you won't get sharp scans. The glass platen in a flatbed scanner is pretty durable, but the interior optics in any scanner aren't. The mirrors have their reflective coatings on top of the glass, not behind it, to avoid ghost images. Those coatings are very easy to scratch.
If you're good at mechanical assembly and disassembly and know exactly how to clean first-surface optics, go for it. If you don't, you need to hand this off to a trained professional. Don't email me asking me for tips or advice. I'm not going to be the "enabler" on this one.
Unfortunately, because I can do all the stuff myself, I don't know the names of good repair people to send scanners to for proper cleaning. Do any readers have recommendations?
Ctein's regular weekly column appears on Thursday mornings. We're on summer hours, so for now that means late mornings.
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Original contents copyright 2010 by Michael C. Johnston and/or the bylined author. All Rights Reserved.