Sigma MC-11 not fully opening lens aperture.

[zach890]

I just got the sigma mc11 EF lens adapter and am using it with an A7ii and the Canon 50mm f1.8 II version lens. For whatever reason the lens won't fully open its aperture, the camera reads it as f1.8 but I can visually see that the lens is always stopped down to what looks like about F2 or 2.2. Even during picture taking the aperture doesn't quickly open fully and stop back down.

Any thoughts? Is it just an issue of compatibility with the mc11? I can't find anyone else who has noticed this on google.