[Brightcolours]

Makes me want to try this now, although a bit late at the moment! Thinking more I may have been a bit quick to blame a "bad lens", although in the use I've had so far from my 50 Art I hadn't come across anything like that. The nearest is some effect in the corners, but I've not tried with high contrast (very bright highlights) type shots so that might expose weaknesses I haven't seen yet.

 

I probably should get more sleep...

 

Edit: added the test shots I did earlier. No diamonds here, but as said these are lower contrast. And relative defocus may play a part in it?

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.