Hi. I have the above lens. I noticed that after taking a photo, I zoom in and can see the centre is focused, but the edges (all 4) are blured and out of focus. Its most noticable when zoomed out (18mm). I treid my kit lens 18-55mm with the same settings and same shot. The photo is focused all over. Why is this so? Is my 18-200mm lens faulty? or needs calibrating? Thanks!