I have a Durst VC670 recently purchased for 6x7 negatives. Everything works well except the light box, when set for 6x7 (as opposed to 35mm), appears to create a brighter 'inner' rectangle when projected without a negative on the base board. I would guess the inner rectangle is about 85% of the full area and is in proportion to the shape of the light box. Needless to say, there is a noticeable lightening of the print edges which I currently have to dodge to bring back into line.

For those not familiar with the VC670, it has an adjustable lightbox that directs illumination through an opaque screen and down a rectangular silvered 'tube' before it hits the negative. It seems to me that the tube is causing some kind of interference in the light path but would appreciate anyone else's suggestions.