There is a issue with digital cameras that makes light falloff at the corners and edges worse, the photosites have depth to them, so the outer photosites are partially shaded and do not receive as much light as the grain at the outer edges of film. The signal from the outer photosites is amplified to account for this, and that creates more noise and less resolution at the edges and corners.
I read an interesting article about the new Leica M sensor that seems to better addresses the sensor depth issue. I the corners. I hope this is the direction for all the big DSLR makers over the coming years.