Shapes of confusion and bokeh: effect of diaphragm shape on out-of-focus areas

A single point in the scene that is not in focus shows up as a blurred circle on your photo. This is sometimes called: circle of confusion. The smaller the diaphragm, the smaller the circle: that's what the diaphragm is for! Often it is not quite a circle: it depends on the shape of your diaphragm. Most diaphragms are approximately circular in shape, yet their precise shape has repercussions for your picture. My Praktica has a hexagonal diaphragm that is quite common in such SLRs. Look at the following blown-up detail:

Hexagons of confusion (taken from a out-of-focus corner of a close-up picture)

The shapes are especially distinct when a very small highlight is out of focus. This situation occurs for example when your flash hits some small object that lits up very brightly.

My Canon A40 is an entirely different matter. It has two diaphragm shapes: one circular and one approximately oval. The shapes of confusion, though, look very odd and are sometimes a little distracting:

Multiple concentric circles of confusion (taken from flash picture)

I am surprised that the shape is not smooth, but contains bright concentric rings. Is this some kind of diffraction or interference effect? Once you know what to look for, you find these shapes of confusion in many of your pictures.

More concentric rings of confusion (taken from a macro shot with an out-of-focus blade of grass in the background)

The look of the out-of-focus areas is called bokeh. Next to these two variations, there are various others. Some lenses give rings in front of the focal point (brighter around the edges of the circles) and smooth circles behind the focal point (brighter in the middle of the circles), or vice versa. The optical theory behind this eludes me. I have one lens with this characteristic.