Originally posted by J2R I'm not sure I'd want to be fiddling around with shims, to be honest. Is it tricky to do?
Well, you kinda have to when you change a screen. Screens vary in thickness, so shimming is how you account for the variance. It's not hard, but can be tedious. The shim goes behind the screen. Trick is finding the exact right thickness shim. It's just lots of trial and error. Pick one to install, confirm that the image is in focus through the VF, take a shot and zoom to confirm, or preferably, view them on your monitor. If it's not right, choose another one and keep track of the ones you have used. If you're not lucky enough to hit on the right one quickly, you'll at least be able to figure out in which thickness direction you need to go, based on what you've tried. The fs.com ones come with several sizes and the right one is most likely in there. If not, you can get a set from Pentax and they are reasonably priced here in the states, less so elsewhere.
Edit: Forgot to mention that when doing these test shots, you need to use your fastest lens, set wide open. You want to eliminate as much of the DoF error margin as possible. The s-screen can discern differences down to f/1.4, possibly even 1.2. Again, this is really not hard to do, it's just tedious unless you get lucky and get the right shim right off the bat. Also, work in as dust-free an environment as you can or you'll need to clean your sensor and/or mirror when you're done. A dry-cleaning with a sensor brush is sufficient since it will just be loose dust particles.