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.
* Image of Back Side of Main Mirror
The image above shows the top of the mirror box when the main mirror is flipped up. The white spots in the red circles are excess lubricant oil.
Oil Lubricant Used in Cameras
Measures are being implemented to control the amount of lubricant applied during manufacture in order to reduce the occurrence of oil spots. However, because this lubricant is required to ensure proper operation of the camera’s precision mechanical parts, it is impossible to eliminate all instances of oil scattering. If you require CMOS sensor cleaning, please contact our Customer Support Center.
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