Archived from groups: alt.comp.periphs.dcameras (More info?
We need some more clues. Is it bright or dark? Always in the same
spot or does it change depending on where the sun is? If so - it's
probably lens flare, and the suin does not have to be 'inshot' for lens
flare to happen - use a lens hood, or shield the camera lens with your
hand to prevent sun striking the front element.
Is it always the same shape and size, does it change if you are
focussed on something far away rather than close?
Have you tried shining a light into the lens to see if there are any
objects or marks on internal elements?