OK, got home, out on bail now .. they're still sus on me.
But as soon as I got home I popped him on my trusty Yellingbo Joe titanuim tripod I recently got off ebay for just $29.99(can't believe how cheap and light their titanium products are) whipped out my trusty resolution chart and WOW!
Absolutely razor sharp dead centre.
Just a small amount of fringing, as I suspected there would be .. easily worked in post.
Here's the image no pp just straight conversion to jpg.
Sigma $s_grid.jpg
You'll notice that the $ sign in the centre is not a black ink rendering, it's actually missing!
The lens is so sharp in the centre it physically cut the paper when taking the shot .. so the black $ sign is a cutout.
The $ signs are actually printed in a 18% grey rendering to show lens aberrations more clearly.
Wasn't expecting it to be so sharp.
But there is minimal fringing as you can see, and some weird distortion too, kind of moustache like but it also has some double rendering effects where it appears to add info into the scene that may not be there.
I'm thinking could be moire, will have to do more testing.
Interesting to see that my distortion results differ from Am's results. No doubt sample variation between lenses.
Anyhow, below is the edited version with a simple one click CA removal routine added in post ...
Sigma $s_grid_CA removal.jpg
You can see that for portraits, it works wonderfully.
Perfect central sharpness and the small amount of CA is easily smoothed out and eliminated.
Will have a quick try at macro too, but it's working distance is a bit of a problem, being only 5 cm or so from the front of the lens. I reckon it may suffer from focal length shortening too.
Not holding out too much hope tho, I have 62, 67 and 77mm macro lens accessory diopters(and filters in general) and the lens seems to use 72mm drop in types .. so an other additional expense in needing rare and hard to find 72mm filters.