With the exodus of Canon users to Sony's Alpha series, adapters have become incredibly important, particularly the Metabones V and the Sigma MC-11. Photographer Luis Gabriel Gerena has done a comparison and produced some slightly alarming — albeit very limited — results regarding sharpness when shooting with a wide angle lens.
It's worth keeping in mind that Gerena's results are based on just one comparison and one lens, so here's our call to action: if you have access to both the Sigma MC-11 and the Metabones V, please perform your own test with your widest lens and post the results below.
It's possible that Gerena's version of the Metabones V has a very small but influential defect. Adapters might not contain any glass but there is a very fine degree of precision required in their engineering and a tiny inaccuracy in the flange distance (in a digital camera, this is essentially the distance between the back of the lens and the sensor) could produce this problem — something that would become more pronounced at wider angles because of the severe angle of incidence.
Usefully, Gerena has provided high resolution and raw versions of his test shots for those that wish to peer at the pixels.