Originally posted by wlachan The # of blades determines how the shape of bokeh looks like when stopped down, but whether the bokeh would be smooth or donut like is determined by the actual optics.
Originally posted by Lowell Goudge There was An intresting thread .. showed the difference between mAny lenses in the harshness of bokeh
it did not address the impact of round vs octagonal or hexagonal aperture
AFAIK it's a combination of blades, optics, and distance. I recently shot my ancient T2 Vivitar 200/3.5 with 18(!) blades, forming a perfect circle - and when focused on a close subject, the glittery backdrop was all big perfect circles. Bokeh balls, I call them. Shooting from a greater distance reduced their size, but they're still balls.
And a couple weeks ago I shot my Schneider Betavaron 50-125 enlarger zoom. Very strange lens, fixed-focus - you adjust for distance by zooming, so framing a shot involves dancing around. It has just 5 blades and they form a star-shaped aperture, but the bokeh is always smooth and formless.