Folks shooting Sony's full frame E mount bodies want choices in glass and while Sony is dramatically investing in new glass, many buyers are coming from a different system. If you have Sigma Canon EF mount lenses and would like to use them effectively on a Sony E mount full frame body, you might just be in luck with the Sigma MC-11.
Sigma designed the MC-11 to allow their Sigma Canon mount lenses to be used on Sony E/FE bodies preserving aperture control and autofocus. They also wanted to be sure that there was no cropping of the image circle on the FE bodies, meaning that a 100mm macro mounted on the MC-11 delivered the proper image circle on the Sony sensor.
So that's what they did. I've had many conversations with people who have switched or added Sony full frame mirrorless to their kit. There are a lot more Canon EF mount lenses out there than there are Sony FE mount lenses. By adding the MC-11 to your kit, you can use your existing Sigma DG, as well as some Sigma DC EF lenses on the Sony FE mount bodies without losing.
Alternate OEM lenses may work as well, but there are many reports that focus speed and overall performance may be degraded. Please note that Sigma specifically only lists certain Sigma lenses as supported. If you use something else and lava comes out, it's your fault.
The MC-11 is best leveraged with Sigma's USB dock to keep lenses up to date, as well as to update older Sigma lenses to make them compatible with the MC-11.
The MC-11 is very simple. It has an EF mount on the front, an FE mount on the back, a lens release button and a USB port for updates. Think of the MC-11 as a translator between Sigma control instructions and Sony control instructions.
In my limited testing, it worked pretty darn well. Because I could not do a with/without comparison on the same Sony a7 II body, it is hard to judge if the MC-11 slows things down. It seemed quite perky to me and it allowed me to mount three different ART lenses up simply and effectively. There is an LED that glows green if ok, flashes green if the device needs new firmware and shows red if the automation is not supported.
I understand that Sigma cannot say outright that the MC-11 can be used with any Canon EF mount lens, or even that it works with both Sigma EF and Canon EF mount lenses for sure. It would make the buying decision that much easier for folks moving from Canon to Sony, who did not have Sigma's own Canon mount glass. While this may reduce the number of potential buyers, that does not diminish the value proposition for those who do own Sigma EF mount glass who want to use it on Sony FE bodies. Potential buyers should check with Sigma's website to be sure that their lenses in question are supported by the MC-11.
Now read this carefully. Sigma DOES NOT say that you can use Canon EF lenses on Sony FE bodies with the MC-11. They are very specific that this is NOT SUPPORTED. But, if you try it, and it works, well isn't that a pleasant surprise. If it doesn't work, don't call Sigma and don't call me. One might imagine that as I had a Sony a7II, an MC-11 and several drawers of Canon L glass that I might have tried using such a combination. I might have tried a 24-105, a 100 macro and something a little longer, like a 70-200 and I might have found that they all worked beautifully on the Sony a7II with the MC-11. If I did such a thing, I absolutely could not tell you that it's safe to buy an MC-11 to put your Canon glass on a Sony FE mount, because that is not supported. Even if it did work.
I have tested a number of Canon to Sony mount converters. I give Sigma credit for being very clear about what they say works and what they say does not. Most other offerings have a "maybe" buried in their specs, including some products costing over $1000 that advertise ideal performance, that in fact work rarely and when they do, the function is glacially slow. Sigma keeps the supported list tight and that is good news for buyers. The MC-11 does what it says it does and is very reasonably priced at an MSRP of $329.99 in Canada. That's a lot less than Metawhatever and this mount does what it says.
In fact if you have a Sony FE mount body and want some absolutely killer fast primes, Sigma is even offering kits of some of their ART series primes bundled with the MC-11. Compare these to other options and I think that you will find the Sigma solution to be a better value.
It's a rather specific use case, but since I tested the MC-11 to validate its suitability for that use case, I have to say that I was pleased and impressed and recommend the MC-11 for its recommended purpose. It may even work for an unsupported purpose, but that would be on you.
I like the MC-11 a lot, for functionality, performance and ease of use. If I was going into something like a Sony A7s Mark II for landscapes, portraits and low light, I would be looking at a Sigma Prime + MC-11 Bundle. If I were looking at getting a Sony A7r Mark II for speed and awesome 4K video, I would also be looking at a Sigma Prime + MC-11 Bundle. You can see a pattern here I think.