I examined my RZ67 Pro II back assuming that it’s similar to the RB67 back. When viewing the film opening with the dark slide removed, it’s easy to see that the pressure plate is about a 1.5mm deeper than the deepest edge of the short ends of the film aperture. That provides plenty of clearance for light projected at an angle to pass under the ends of the film aperture and strike the film beyond the usual frame boundary at the ends of the frame.
When image-forming light from the lens is projected back through the film aperture, the maximum angle that light can travel from the rear lens element passes through the aperture and defines the image rectangle on film. This is always about then same size due to the same (or nearly the same) projection angle of the various lenses that fit the camera.
But in the case of spill light reflecting from the interior of the mirror box past an out-of-position light baffle, the light can travel from the sides of the mirror box past the edge of the baffle obliquely to pass under the ends of the film aperture due to the 1.5mm clearance between the end of the film aperture and film lying against the pressure plate.
The source of the light is most likely from the lens to the mirror to the focusing screen. Some of the light that strikes the focusing screen must reflect and spread sufficiently to slightly illuminate the interior of the mirror box during viewing. Normally this is no problem if the light baffle is sealing the opening to the film holder as the designer intended. But if the baffle is not fully closed, then some light from the mirror box can bleed thorough the gap between the baffle and the chamber to expose the film—including a small area beyond the usual border due to the clearance mentioned. And, since the light still grazes the edge at the end of the film aperture, this would account for the sharp boundary, even into the area beyond the usual frame boundary.
Last edited by Ian C; 10-27-2012 at 04:38 PM. Click to view previous post history.