I think the physics of sun-stars is roughly as complex as the physics of bokeh. The aperture, the glass itself (optical design, maybe coatings?), the focal length, the distance to the point of focus, the distance beyond the point of focus to the light source, the light source itself, etc all play a role. I know that shooting directly into the sun I usually need to stop down as far as I can get to see a star and it's still usually pretty indistinct. Here are two of the sun taken with the same 25mm lens, same tiny f22 aperture, apparently at roughly the same distance, through somewhat different atmospheric conditions - one is quite diffuse and the other quite sharp.
Stroud-89-Edit by
Ray, on Flickr
Stroud-18-Edit by
Ray, on Flickr
Is the amount of moisture in the air the difference? Beats me...
But here's one shot at f8 also, with an incredibly distinct little star in a reflection off of the top of this guy's horn - you can't really see it at this size, but if you click through you can view it as big as you want in flickr and it's a really cool little detail in this shot... This is at a much longer focal length (somewhere between 75-150mm) with a much closer subject and a much smaller point of light. For whatever combination of reasons, it worked really well at f8 and I suspect would have at f5.6 also...
NYC - day 5-112-Edit by
Ray, on Flickr
Sometimes you catch them just right, sometimes you don't... I think all of the stars have more points than the lenses have aperture blades though. The top two considerably more. This bottom one maybe not but I think some of the points are obscured by other light coming off the horn... And way more in Peter's shot. I don't know of lenses with more than nine aperture blades...
-Ray