[popo]

You don't get astigmatism shapes when you are that defocussed. Test for astigmatism with in focus or nearly in focus light points at borders.

 

Sigma 50mm f/1.4 A test by Crestie Crazy, on Flickr

 

Here's one of my first tests when I got the lens. You will probably want to fight through flickr to get the full resolution to pixel peep. A starfield is one of the worst case scenarios when in focus, and there's not a lot going on even at f/1.4 until you pass roughly the APS-C circle, and even at full frame corner there isn't that much. Note I used a static mount for this test shot so a tiny amount of trailing may be seen and should be disregarded.