[Green_man]

The number of rays is determined by the number of aperture blades, as well as whether there's an even or odd number of aperture blades

https://phillipreeve.net/blog/best-lenses-for-sunstars/#The_...

Interestingly, some modern photography lenses have achieved aperture mechanisms with much rounder geometry, sometimes with near perfect circles at multiple apertures. This can result in a more desirable bokeh, at the cost of well defined sun stars.

[blauditore]

The article fails to explain that, but round aperture is equivalent to infinitely many straight blades (and thus rays), so there is a light halo instead of distinct rays.

[Sharlin]

Which is essentially also why you get more diffraction limited as you stop down the aperture -- a larger and larger fraction of the light that gets through passes near the edges (and gets diffracted) rather than the central area.

[hatsunearu]

yup, but when you step down the aperture a lot (which is when the sun star effect becomes more pronounced), the aperture transitions from more circular to more polygonal, so with a lot of lenses, the behavior is that when your aperture is a few steps of fully open, it is basically circular but when you need the sun starts it's definitely there.