Bokeh

It is most often advisable that you choose the background color or texture to differentiate the subject. The blurriness behind the scene is called "bokeh." It is not French, but Japanese.

Here, just when I thought that DA * lenses are crap, it reminds me its optical capability. This was taken at wide open, and pretty sharp although you are right, this is right at the center of the lens. From the framing perspective, you want the stupid bird slightly off the side, but then she would not be as sharp. But there is nothing wrong with not being sharp. Photography is very confusing.

Pentax K20D
DA * 50-135mm
1/125 @ f/2.8
0 ev
ISO 200