I have tested the lenses on my Canon 1DsIII (21 megapixel, fullframe). The lenses were mounted on tripod; I have used mirror lock up and self timer. The following images are 100% crop from the unprocessed RAW file. The test subject was a big library (about 4.5x3 meters).
Canon 14mm f/2.8 L USM II (vs Sigma 12-24mm and Canon 16-35) | JuzaPhoto