[GeneV]

If diopter adjustments didn't do the trick, I'd be reluctant to pursue any shimming until I was completely sure it wasn't a technique issue. OP needs to do more practice with manually focussing in low-light, and shoot a lot more scenes, to make sure it is not just a technique issue related to poor illumination, dim viewfinders and perhaps tired eyes.

If I was a eye doctor, I'd be reluctant to immediately advocate eye surgery if someone came to me and said my vision is sometimes a bit fuzzy. That's what some of these shimming and other suggestions sound like to me.

I'd agree with that more if we were talking about serious surgery on the camera, but shimming is pretty easily done and undone. Still, the diopter is the best place to start. I've had trouble focusing and found that the slider for the diopter had moved.

[Ron Kruger]

Hey Rollo,
I don't know if it is generally accepted, but this is the simple way I adjusted my diopter:
I put the camera on a tripod. Then I took a very simple scene with a clearly defined (stationary) subject and contrast and autofocused on it. I took a pic on this AF setting, then adjusted the diopter to agree with this AF point. Next I switched to MF and spun the lens off of focus, focused on the same scene manually and took a pic. Then I compared both the AF and MF shots on my computer at 100 percent to be sure both were accurate. If you want, you can do this same proceedure with LV.
Not very scientific, but it worked for me.
One of my only complaints about the K20D, is the diopter clicks instead of sliding. For some people, their eyesight falls between the clicks. I would much prefer a sliding diopter with a lock.
I have a K7 on the way and hope they have improved the diopter setting, but doubt it, given that the majority of the market have a P&S approach.

[PocketPixels]

Ok, manual focussing is something you have to learn to get right, but if it is you and not you're camera/shim, you should expect a more or less even number of FF as BF.

Quite right. I moved to a new city shortly after this post, and didn't do much low-light shooting for a while, but when I went to a new poetry slam a few weeks back, I used my 50mm manual focus lens at f/2 and f/2.4. I took maybe 50 photographs. I took Marc's advice and paid special attention to making my subject not just "in focus," but "the MOST in-focus" part of the shot. I got no keepers from that whole night. Partly the light was impossible (very dark room, too-bright spotlight with a chrome-plated microphone stand (WHY ON EARTH...?) to dazzle my sensor), but also nearly every shot was poorly focused. Not once, in fifty pictures, was the region of greatest focus in front of the subject. I got three okay pictures and the rest were all back-focused. If I were just bad at manual focus, I should have front-focused some of those, right?

I have changed the diopter here and there, and I believe I have found the correct setting for me. The suggestions about looking at white walls during diopter adjustments were smart, but in the end I was already properly dialed-in. So it's either the lens plain sucks at open apertures, or I need a new and shimmed focusing screen.

I'm going to buy a split-circle focusing screen that comes with some shims. I'll let you know how it goes.

Thank you all for your advice.

[pentup]

The most important thing I take away from Marc's post is the realization that, in naturally enough concentrating on my subject, when my eyes decide it's "in focus" I may very well be failing to observe that a foreground/background item might be more in focus. Anyway, I've found that putting the O-me53 magnifying eyecup on helps a bit. (partly because it allows easier access to the diopter adjustment )

Looking at the OP's photos, I wonder whether trying this might help: When I'm not sure of focus, sometimes if I can concentrate on a small highlighted area - for example, a highlight on one of the peoples glasses, in this case, or the highlight in someones eye, and I try to get them as "undiffused" looking as possible, this helps more than trying to perceive "sharpness." Or if there's a small piece of text, a sign for example, in the plane I want to focus on, I look for the loop in a letter "e" for example, and try to get it as "tight" as possible.

All this because I'm too lazy and cheap to get a KatzEye.