I shot my bedroom window screen in macro mode to see how the corners look close in. The issue is not so much blur as it is unbelievable amounts of chromatic aberration. As shown here:
Click through to see it. Here are some crops, also better viewed at the full 800px size.
I do need to exchange this one.
I shot this image of Karen at 80mm effective with direct flash inside in very low light. The F300 at 800 ISO is pretty darned good in my opinion.