[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?

What exactly causes the cut off in the shapes at top/bottom near the top/bottom edge? 

In one of the images you see the bottom being cut off when the highlights are near the bottom and the top being cut off at the top of the image, and the other image shows the top being cut off near the bottom of the image and the bottom being cut off near the top of the image.