Lens maker Carl Zeiss has published the latest of its quarterly Camera Lens News in which they discuss the question and the mathematics of bokeh, the term used to described the blurry quality of the point of lights.
The article, an excellent piece of writing, not that I understand the technical part of it, explains in depth the issue of bokeh and the mechanics as well as the aesthetics behind it.
Rusty fence bokeh
Even if you are not technically-inclined, you’ll come away more enlightened after reading the article. You can download the bokeh “white paper” here. Read it and get a sharper view of the blurry issue 
