I had recently returned my 5D3 to Canon due to permanent sensor dust spots (8 spots) visible at f22 to f8. Canon service confirmed that the spots cannot be cleaned externally and were internal to the sensor. Canon replaced the sensor and upon receiving the 5D3 back, I was surprised that there was still one spot visible at f22 to f16. This makes me wonder what is Canon's manufacturing specification for dust spots on their sensor? I would think that the sensors are assembled under clean room environment and should have strict particle control.