Tim, this is on my 360 Gerogon on the 8x20. I does not look like it will cause any vignetting but I have not shot with it yet. The nice thing about this is you can shoot with or without a filter. I have to put a dark sock, if you will, over the lens to keep stray light from getting in the back. I can adjust it for just about any of my lenses without a shutter. I'm going to use it for the 8x10 and 8x20 systems and I will make one for the 4x5 and 5x7 systems also. The Packard has the 3" opening and it adds about 14 oz. to the front.
Jim