Possibly a dumb question, but here goes:
I have a canon-converted high contrast screen for K5 from focusingscreen.com, the same as in the photos above.
I have mine in the camera with no shims at all.
When talking about front/back focusing I assume you are referring to difference between what the eye sees as focused through the viewfinder, and the actual recorded image being focused in front or behind that point. Right?
I ask because as I said, I use no shims, and my tests seem to show that what I see through the screen, and the resulting photograph are as identical as I can perceive.
http://oh-hi.info/tech/00043873a-768px.jpg
*However* I did notice that the camera's focus confirm lights seem to front focus. I tend to ignore it so much that I have recently turned it off entirely, so maybe it was actually back focusing. But in either case, the focus confirmation lights were incorrect when visually and photographically everything is working correctly...without any shims.
Are the shims so that I can see focus correctly? Or so the focus confirmation can see it correctly? Or both?
Does this mean I got lucky with not needing shims? or does this mean everyone else didn't get so lucky? Or that I need my eyes tested :)
... or that they are putting their screen in backwards and thereby needing shims to compensate? (thats not really a serious question, but it did appear that they could go in both ways when last I checked)