Optical Weirdness

[kafin8ed]

Some of the photos I took recently have an arc of lightness through them and I'm trying to figure out what is causing this. It is always in the same place. You can see it pointed out in the #1 pic here. The #2 pic doesn’t have the arc. Both pics were shot with the Canon 1DmkIIn/70-200f2.8L/1.4XL teleconverter @ f5.6 and 280mm. #1 had a 1/2500th shutter speed and #2 had a 1/400th. For #1 I had the sun at my back—I don’t think this could be flare, # 2 was shot much more towards the sun. I got the effect in numerous images, but it was most noticeable on shots with sky. It should also be noted that the effect rotates with the camera. A Vertical shot is attached as image #3 which was shot at f5.6, 155mm, 1/1600 with the sun to my back. Also, this proves that the effect is not focal length dependant. Thankfully, most of the images with this weirdness were pretty dull and boring but I’d like to figure out what went wrong before I actually get a good shoot. Any ideas??? Thanks!

-Alan