In a service notice, Canon notes, "We have discovered that images taken by some EOS-1D Mark III and EOS-1Ds Mark III digital SLR cameras may exhibit ring-shaped spots.... These ring-shaped spots are the images of oil lubricant on the surface of the low pass filter (LPF), which is located in front of the CMOS sensor. This oil is used inside the Mirror Box to lubricate some of the camera's moving parts. If the oil is applied too liberally when the camera is manufactured, it can become scattered onto the surface of the LPF through normal camera operation."

If you've experienced this oil spotting or have seen oil on the surface of the back side of the mirror, contact Canon's Customer Support Center. It will arrange free cleaning of the mirror box.

Canon U.S.A., Inc. Customer Support Center:
800-828-4040
carecenter@cits.canon.com