Originally posted by Eruditass It's mostly due to the lens design build, not the aperture shape (since this doesn't matter when it is wide open). All lenses will have a round aperture when wide open, but only those with sculpted blades/many blades will have a round aperture when stopped down at all.
Typically, designers that put much care into the aperture shape will also have good bokeh (both wide open and stopped down).
Blende8 gave a good example of what I am looking for.
The lenses in this club will be scattered throughout different makers.
I have a 200mm Takumar with 15-16 blades.
The types of things I suspect that people would post are the OFF details like Blende posted, and also the starburst you see from difraction interference around the minor discontinuities at the intersection of the blades.