I have noticed in some photographs that the bokeh has a 'spherical' quality, like the background is curling around the subject. As in this picture "The image has been taken from wikipedia.org and the original author is carlosluis. This file is licensed under the Creative Commons Attribution 2.0 Generic license. In fact, I've even taken a couple shots with my Sigma 30mm f1.4 that have given me this effect, but I don't know what I did to get it. What causes this effect, and how do you intentionally setup a shot to reproduce it?