So tonight I was shooting more flowers, and I noticed yet again that most of them didn't seem as sharp as I thought they should be. I was shooting with the lens stopped way down for maximum depth of field (where maximum is probably something on the order of a quarter inch, I'd guess.) I had a vague notion that the shots where I had the lens open wider, so I did the geek thing and experimented.
What did I learn? On my lens, maximum sharpness is around f/11, after which it starts falling off pretty badly. Here's one set of test shots from tonight. Clicking on the image will get the large but very detailed version, which shows a lot of detail.
When you look at the picture, bear in mind that the thing you're staring at is about a millimeter across.