|Zeiss 50mm Loxia - Looking at art, looking — Gallery of Modern Art, Brisbane|
After reading reports that some lenses close down, focus, re-open and then take the shot (sometimes causing a mis-focus), I decided to check out the Sigma 24mm and 85mm Arts while attached to the Sigma's MC-11 Mount Converter.
One of the assumed benefits of mirrorless cameras was that they were exempt from the front and back focusing errors to which DSLRs can be prone. So, it seemed a useful question.
I'm pleased to report that from what I could see in my tests, neither the 24 nor the 85mm Sigmas close down on focus, on either the A7RII or A7II.
I'm presuming that this will apply equally to the upcoming Sigma 135mm, f/1.8. I'm also guessing the 135 is due soon as there is a firmware update for the MC-11 to accomodate it.
And a final note: I'm not sure many people "get" the MC-11 Mount Converter. It's not just a tube making a generic connection between Sony E-Mount bodies and applicable Sigma EF Mount (or Sigma SA Mount) lenses. The MC-11 applies E-Mount firmware to Sigma lenses that have been designed to be able to directly accept that information. That's why, for example, the MC-11, provides Sony's eye auto-focus that other adaptors do not.
It's true that the MC-11 will work with some Canon glass. But it's sad to read reviews marking the MC-11 down because it doesn't do those non-Sigma lenses as well as it does the Sigmas. That it works with any other lenses at all is a bonus, icing on the cake, an Easter Egg.
Here's the short version: I believe that the Sigma 85mm Art with the MC-11, on an A7RII, is the best lens on the best body. I think the combination uses every bit of Sony's 42.4 megapixel sensor, provides (reasonably) fast auto-focus, eye focus, and throws in the 5-axis stabilisation that Canon and Nikon users can only dream about — usually for less money than comparable Sony or Canon lenses.