I just got back around 12 rolls of film shot in 6x12 format that are 95% out of focus, and i've also developed my 4x5 BW from the same trip and they are also out of focus. All film was shot on a homemade field camera (6x12 with a roll back).
The back on my homemade camera is actually a Toyo monorail international back that has been mounted onto a wooden frame.
I'd been using this homemade camera for 8 months or so successfully until I recently broke the ground glass. I replaced it with ground glass from a old speed graphic and shot a coulple of rolls of film and the odd 4x5 and had no problems, though I found the speed graphic's ground glass significantly darker than the previous Toyo one. I recently bought a Sinar back off ebay (same as this one: http://cgi.ebay.com/Sinar-P-4x5-Film...QQcmdZViewItem )
This is for another project, but in the meantime I took the ground glass out of this back and installed it in my current camera to replace the speed graphic glass. I didn't test this new setup before my last trip, but didn't see any reason why the camera would perform differently with this new ground glass (other than hopeing for a brighter image)
The only other thing I changed in my shooting technique was I used a Toyo GG magnifier which I recently bought instead of a cheap loupe that I'd been using previously (the cheap loupe was higher magnification, and cost around $8). I found the Toyo much better to use - I could see closer to the edge of the frame as have a smaller diameter than my older loupe, and the whole circle was in focus, where as my old loupe only the centre was in focus, so it was hard to check the edges of the frame.)
I'm pretty sure it's not user error as although I still get shots that aren't always sharp from time to time with focusing errors, I never get 95% of out of focus - especially with the roll film stuff as a lot of my 6x12 stuff is often just infinity focused anyway. Most stuff I get that is out of focus is when I've used tilts or swings and haven't got the focal plane quite right.
Some of the shots have in focus areas - mostly the ones I'd set up with tilts. I'm sure that these infocus areas did not match what I was seeing on the ground glass when I set the shots ups (for example in focus foregrounds but out of focus infinity which I was sure was sharp on the ground glass).
I'm at a bit of a loss to what may have happened here - the alignment between the ground side of the glass and the film plane should not have changed as there are no shims to be adjusted on the toyo back, just two clips that hold the glass onto the frame that are tightened be screws. The glass is installed with the ground side facing the lens.
I'm going to do some test shots with some polariods when I get the time (in the next week or so) but I'd be interested if anyone can spot what the problem may be.