I've been taking some 6x17 shots on a linhof. The pictures are sharp in the cen tre but very unsharp even to the unaided eye at the edges. I can't really belie ve its the schneider 90mm lens or schneider centre spot graduated filter so coul d it be the back plate or other film non-flatness issue. The pressure plate has a different setting for 120 and 220 and it appears to be correctly set for 120 but the instructions are in german and I don't speak any so could have got it wr ong. Any ideas?