Originally posted by Raffwal Thanks for the wiseass comment rawr. However that is not what I was asking about or for.
An example of my experience with my K-5: I have fine-tuned the AF using natural light, a good high contrast object and a tripod. And in good light the results are somewhere in the league where they should be. But just last evening I was shooting in the backyard, using the centre point focused to a thick bright white lamp post against grass. A good high contrast object, isn't it? Well, not according to my K-5 which decided to focus about a metre behind the post.
the problem with focusing on edges with nothing behind them is that the camera can be left unsure which of the two things it should focus on.
a post edge is sharp true, but so is the grass behind it, all the little blades make a great focusing grid.
auto focus is not perfect, and lacks a vital function, mindread.exe, where the camera focuses on what you are looking at. canon tried this once with an eyeball sensor such that focus detectors changed with the polsition of your eye. Not sure what happened to it., but any way. focusing works best and reliably when focusing on a surface with texture, not an edge with a distant subject behind it.
edges are great for split image focusing, but the camera does not work that way