One of the little bits of wisdom I have picked up along the way is that for a lens to have optimal correction, it needs to have the rear nodal point somewhere in the vicinity of the lens. With the 40 something millimeter flange to focal plane distance that most 35mm SLR's have (this includes the istD), getting a super wide with everything excellent is going to be a very difficult proposition. Generally, really short focal length lenses are not the best performers.
Rob mentioned a while back that the Mamiya 7 43mm lens was a much better performer than the Pentax 6x7 45mm lens. Not having a moving mirror in the way opens up all sorts of possibilities to the lens designer. William Robb
