I’ve been testing the Sony A7RII with the Sigma ART A Lens. And getting some fantastic results. So far, this is an amazing 35mm Prime lens. Especially cheap considering in is F1.4 prime. Yes you need a A mount lens adapter. But it’s very sharp and vibrant.
But my first initial comments.
Focusing is pretty fast. The Lens is very sharp from f1.4 to F22. It has a very nice quality. Very little vignetting. But enough to give it some personality. Sharper at the edges that i thought it would be. For the quality, this is a very cheap F1.4 35mm lens.
The lens is big and heavy, and not a match for the Sony FE 35mm F1.4 Distagon. But for half the price. Its great value for the money. And if you don’t mind a non native lens, this is a fantastic Fantastic lens. Gorgeous Bokeh, lovely DOF. And a great fast wide lens.
Rickyban Photography 
How would you describe the auto-focus compared to a native lens?
If you use the LA-EA3, you will get the full benefits of the A7R11 autofocus, apart from eyeAF. It’s not as fast with the larger lenses like the 70-400 compared to the notice 70-200. But i’s quick with the 35mm and there is not much in it. Just avoid the LA-EA4 unless you have a screw driven focuser. The LA-EA4 uses its own focus control, not the cameras. So it is slow and hit and miss for most A mount lenses that i have tested.
Not a match for the Sony 35? Yeah because the Sigma is better. Reviews, MTF and DXO back that up.
I Have both lenses. And i can honestly say that the real world differences are sharpness. The Sony f1.4 is sharper than the Sigma. There is no doubt there. But the Sigma has a charm to its images, especially when shooting at 1.4. Its much softer than the Sony. Apart from that. I love the sigma.