Originally posted by calsan 6 blade lens will produce the appearance of a 6 point starburst, but it won't be sharp looking. This is because it's actually made up of twelve points - a six pointed starburst closely overlapping another six pointed starburst.
A seven bladed lens will produce a nice, sharp 14 pointed star with all the points evenly spaced.
So - to get nice sharp starbursts, you acutally want a lens with an odd number of blades.
The DA limiteds all have this 'feature' with either 7 blades in the wider lenses and 9 in the more telephoto lenses.
See the photo below - notice that there are actually 12 points from this shot (cropped) with the six bladed 18-55.
The 12 points you see here is actually correct, you get 2 X the aperture blades for any lens, it is just that if the corners of the hexagon for a sox bladed aperture are absolutely perfect, the diagonally opposed lines fall on top of one another and you get only six . Any slight imperfection in the symmetry results in what some might call doublets
This will be a lens to lens variable even within a specific lens model