If you're into photography and have bought a single lens reflex camera, you'll soon start swearing over the fact that there they don't use the same lens mount.
Canon, Nikon, Sony et al all use their own lens mounts meaning you're stuck with one brand that might have great optics, but only so-and-so cameras. Unless you sell the lot and start again, which is an expensive proposition.
There might be a really great lens from manufacturer A that you would like to use on camera B, but it's not going to work, not usually. There have been adapters around for a few years now, but they tend work with just some lenses, or provide partial features for these. Autofocusing accuracy and speed can get compromised severely using adapters for instance.
Having tried the Metabones III (not the newer IV) adapter with very mixed results to mount Canon EF lenses on Sony A7 cameras, I wasn't sure what to expect from the new Sigma MC-11 device.
This little device sits between the lens and the camera body; the MC-11 is officially aimed at customers who have bought Sigma lenses with Canon EF mount and who wish to use them on their Sony A7 full-frame and other cameras like the A6300 with the smaller APS-C sensor.
The MC-11 makes the Canon lenses seem like Sony ones to the camera, and so far, it's been great on both the A7RII and A6300. Sigma says the MC-11 is for its own ART lenses only, and have listed 19 of them as being fully supported by the adapter.
My 35mm Sigma 35mm f/1.4 lens with Canon mount is pretty much perfect coupled with the A7RII: autofocus is fast and accurate, and camera brings the additional benefit of in-body image stabilisation, something I don't get with my Canon 7D.
What makes the MC-11 doubly attractive is that you can use newer Canon EF (not EF-S) lenses with it. I had the brilliant 70-200mm f/2.8 for a while, and can report that it works pretty well with the MC-11.
Am hoping to have more Canon lenses to try out with the MC-11 soon, but indications are that the MC-11 works well with a range those with a few exceptions.
Note that using Canon lenses with the MC-11 is totally unsupported, so you're on your own if you do. Pro tip: make sure you switch off the camera before swapping lenses, for safety's sake.
That'd be newer Canon lenses of course: for the sake of it, I mounted my now-ancient 70-300mm f4-5.6 IS USM zoom on the MC-11 and yeah, nah, it's not fantastic with the autofocus stepping slowly back and forth.
Drawbacks to the MC-11 include the high-ish NZ price, around $500 when it sells for US$249 in American online stores.
Besides, old lenses just don't have the sharpness required for the amazing Sony A7RII with its 42 megapixel full-frame sensor.
There's a USB port on the MC-11 as well, to update the firmware in the adapter to make it compatible with future lenses which is good.
Drawbacks to the MC-11 include the high-ish NZ price, around $500 when it sells for US$249 in American online stores (around $355, thanks high NZ$) but you have to add freight and GST to that, and bear in mind the lack of local support for goods bought from overseas.
The adapter isn't weather sealed either, which some pros might need.
Those flaws are not major however, so if you're a Canon refugee to Sony cameras and don't wish to part with your lenses, the Sigma MC-11 is definitely worth trying out.