[ksk]

While you can easily add the adapter, you are using the lens in a way it is not designed for. It will work for various definitions of work, and casual users might never even notice the difference. But it will never be equivalent to its native usecase.

Some random issues of the top of my head..

1) Tiny tolerance/alignment differences between the target mount and adapted mount can cause the lens to become decentered or be mounted at a tiny angle to the sensor plane. 2) special mirrorless camera features might not work as well e.g. eye AF in sony / object tracking / etc 3) possible autofocus hunting issues on outer focus points or autofocus performance will be impacted 4) pro level lenses like the canon 500 F/4 combined with a 1Dx type body unlocks a faster focusing mode which wont be available with an adapted lens.

[rangibaby]

> Tiny tolerance/alignment differences between the target mount and adapted mount can cause the lens to become decentered or be mounted at a tiny angle to the sensor plane.

It is certainly possible to make an accurate mount adapter; see the Pentax m42 → K mount adapter. The "problem" is people don't want to pay for a good one; the Pentax adapter costs about 2x as much as the adapters everyone seems to buy.

[ksk]

I have had mixed results with the Sigma MC-11 adapter. Some lenses worked OK, and some didn't. I don't have the setup or the resources to do a thorough analysis. I attributed them to the 'slop' in the system - From the focus motor to the mount to the adapter to the AF system and sensor. I was probably unlucky and the +/- tolerances didn't cancel each other out.