i guess it's the problem of the stepping down... (one side) on the other side it guess it's simply the quality of the whole lens thing... good wide angle lenses are quite hard to produce... and the "wider" they are the more do they "bulge" the pic... from that side it's kinda normal... with an fisheye you'd have the extreme... the only suggetion for solution is to crop.... or try the correct sized lens by chance and see how the results are...