One of the better "balanced" lenses on the A7R II. A Sigma 150-500mm shown extended to 500mm and mounted on the A7R II with a Metabones III adapter:
https://lh3.googleusercontent.com/-qPQskfLQaJk/VdjgggLWbII/AAAAAAAAnE4/vwCkpUvanIQ/w600-h393-no/Sigma150-500mm%252CMetabonesIII%252CA7RII-2015Aug22.jpg

Autofocus is actually quite good with this combo; about the same as using the lens on my Canon 60D. Good enough for wildlife; probably not birds on wing, although birds on a tree are ok :). Works quite nicely in low light conditions both on a tripod and handheld. For handheld I normally push the ISO to the 800-3200 range, although I've used 12800 with the combo with acceptable (at least I captured a picture) quality.

All EOS to E-mount adapters, however, are not created equal! The Metabones III has been the best one I've tested todate; works with all of my Canon, Sigma and Tamron EOS mount lenses. I have a Metabones IV on order.

The Kenko 1.4x teleconverter also works with the Metabones III and passes lens info from all my Canon lenses but only works in manual mode with my Sigma and Tamron lenses.

I've also tested a few other adapters. The Commlite adapter does not pass any Sigma lens info to the camera and does not like my older Canon lenses but works reasonably well with new Canon lenses. The Fotodiox adapter, my newest, sporadically hangs with the Sigma lens and older Canon lenses but works quite well with newer Canon lenses. The King adapter, the first one I acquired, works well with my newer Canon lenses but does not like Sigma, Tamron or older Canon lenses. Strangely the Commlite & King adapters work better with my A7S & A7 II than they do with the A7R II?

When an adapter/lens combo is working well the autofocus is about the same speed as (a touch slower than) native lenses on the A7R II. This is a HUGE improvement over adapted lenses on the A7S, A7R and A7 II, where autofocus was slower than manual focus!

Just a few notes/comments...

bwa