5) Larger sensor systems have softer edges and more vignetting than smaller sensor systems
Once again, as discussed in Myth #1, this belief is a result of people comparing systems at the same f-ratio rather than the same aperture diameter. At the same f-ratio, the larger sensor system will have a larger aperture diameter, and thus a more shallow DOF, which will result in the areas of the scene outside the DOF being OOF (out-of-focus), as well as greater vignetting. A more fair comparison for edge sharpness is to compare at the same DOF, or, often even more appropriate, at the lenses' sharpest settings, since it is rare that edge sharpness plays a role in high ISO photography. However, it is disingenuous to compare edge sharpness and vignetting by artificially handicapping the larger sensor system with the same f-ratio as the smaller sensor system.[...]
However, back to the UWA situation, there is another angle to this story. DSLRs with a 3:2 aspect ratio must shoot wider and then crop to match the FOV of a 4:3 aspect ratio, and this cropping all but eliminates the soft corners, if they even exist. For example, for a Canon 5D to match the perspective, framing, and DOF of an Olympus E3 shooting at 7mm f/4, it would have to shoot at 12.5mm f/7.1 and crop to a 4:3 aspect ratio. This would leave 10 MP on the 5D image, which would still match the pixel count and framing of the E3 image, while eliminating the extreme corners. Likewise, the Canon 5DII (FF) has more pixels than the Canon 50D (1.6x), which also gives it more cropping latitude. However, a 50D has more pixels than a 5D, so the 5D would have no such luxury, except if the lens were unable to sufficiently resolve the smaller pixels of the 50D. In this case, a cropped image from the 5D, despite having less pixels, would likely retain the same, or even more detail, in the instances that we would need to frame wider and crop the corners out.
Stopping the larger system's lens down to normalize the DOF has the additional benefit of increasing the sharpness of the lens (especially in the corners) and reducing vignetting. Many 4/3 proponents like to cite their glass as being "sharp wide open" with no significant vignetting. However, "wide open" for 4/3 is "stopped down" for 35mm FF.