I recently noticed a weird shadowy thing on some of my photos taken with my Canon Powershot S30. It only shows up when I am zoomed in (optical zoom) and it's more noticable if there is a solid colored background (sky, wall, etc) It is perfectly rounded, so I'm almost positive that it's not a scratch, and I've cleaned the outer part of the lens, viewfinders, etc carefully to make sure it's not just dirty. Here is an un-edited example of what I'm seeing:
The size and focus of the shadow changes when I adjust the zoom in and out.. when it's completely out (wide-angle) it's almost invisible, but as I zoom in, it gets more and more noticable, darker and clearer.
The camera is a year and a half old, so the warranty is over, unfortunately. I'm just wondering if anyone else has had a similar problem with this or any other camera, what might be wrong, and how I might fix it. Thanks for your help!