I recently got my hands on one of Canon's finest lenses, the EF 70-200mm 2.8L IS II. The lens is one of those white bazooka-like tubes sometimes spotted along the sidelines at sporting events. Mine, in fact, was purchased to photograph roller derby.
The 70-200 2.8L is, in essence, a giant funnel for photons. It is meant to capture action, at a distance, in relatively low light. As a bug photographer, though, I've been experimenting with the lens outside of its comfort zone, focused as close as it can go.
(Yeah, I'm dangerous that way.)
The effect of a wide-open 70-200 2.8L at minimum focus distance is like nothing I've seen. Have a look:
The out-of focus light comprises what's called "bokeh", and each lens confers a unique quality related to the configuration of the internal lenses and the shape of the iris. The bokeh from the 70-200 IS 2.8L is so smooth as to resemble a painting!
Here's another example with the lens stopped down a bit further:
If you're having trouble visualizing what I mean by bokeh, consider the oddly pentagonal backdrop spots from a Canon EF 35mm f/2.0 prime lens:
A different lens, a completely different quality to the unfocused backdrop!