Brett Deacon , Apr 23, 2007; 11:15 p.m.
I'm sorry you're having such an annoying problem John. I have the DAYI 6x17 back which is as I understand it essentially the same in construction and structure as your 6x12 back. I have a few questions/comments that will likely not solve the problem, but it never hurts to rule out the most obvious explanations first.
First, am I correct in assuming from your post that you notice this flare on *every* exposed frame and not just the first or last shot on the roll? I ask this because I've only ever had one fogging problem and this was due to operator error (accidentally exposing the as yet unrolled film at home when removing it from the back). In this case, only one image (of course, the best shot by far) was fogged. I'm sure this isn't the problem you're having, but...
Second, can you please elaborate on which specific portion of this device you're referring to as the "6x12 aperture of the back?" On my back there are only four places where light could conceivably expose the film: (a) through the lens when the darkslide is pulled, (b) through the small side slot occupied by the darkslide, (c) through the small frame counter holes on the rear, and (d) the connection between the 6x12 back and the view camera. Barring some construction defect or additional pathway to the film on your back, I would expect that the following problems could cause a fogging problem: (a) a light leak in your bellows or other part of your view camera, which if present would cause fogging on 4x5 film as well, (b) not fully inserting the darkslide or not leaving it in place throughout the duration of the exposure, (c) a light leak in the exposure counter holes on the rear of the back, or (d) an insufficiently light-tight connection between the back and the rear standard of your view camera. Do any of these seem possible, or is there some other means of light reaching the film on your back?
If the problem involves light striking the stainless steel roller inside the back, you could always paint it with a matte black finish and observe the effect.