What produces the heptagons in this image?

[capo]

Hey there. First of all, hello to everyone! I'm new to the forums.

 

I recently got my hands on a Mac machine and idly sitting there looking at the wallpaper, I got thinking as to the causes of the heptagons in this image (one on the blade of grass, one in the lower-left-center, etc):

 

 

The explanation may be remarkably simple, I don't know. But I ask myself, why heptagons? Why not pentagons, or hexagons, or circles? And what causes this optical effect in the first place?

 

Thanks in advance.

 

 

[John Cuthber]

A very out of focus image of the iris diaphragm

http://en.wikipedia.org/wiki/Diaphragm_(optics)

[capo]

Simple when you know how! Thanks for pointing that out.