bokeh: round or not

[aeronic]

I have a 18-135 efs lens which has rounded aperture blades hence round bokeh balls
However my nifty fifty has five blades. Hence pentagon bokeh, and unless I use the max aperture of f/1. 8 in which case it's round - why??

 

[BacktotheFuji]

At f1.8 the aperture blades are wide open and don't get in the way. As the lens is round, you get round bokeh

 

[Naboo32]

Good question - good answer !

 

[aeronic]

Indeed, thank you.