Two more things for the record (kinda like warranting arguments):
1. You're taking a 1:1 macro if whatever you're shooting top to bottom is about the size of a US Quarter. 25mm, I think. Two quarters side-by-side and you're shooting 1:2 (which is still a super close-up), and so on.
The closer you get to something, the narrower your plane of focus (that chunk of space that's in focus) is going to be. So you could be shooting a landscape at f/8 and most everything would be in focus (at or near infinity). But if you were shooting a flower the size of a quarter - at minimum focusing distance at f/8 - you might only get 1/16th of an inch slice in focus. And as you back out, that slice becomes wider (no matter the f-stop).
That's probably the reason that the focus drops off near the bottom of that leaf - it's behind that slice of focus. See that bee of mine that Dirt posted. That's very nearly a 1:1 shot (maybe 1:1.4 or something) and I'm guessing I have 1-2mm of focus slice in it (wide open at f/2.8).
