Dirt on the sensor!
The camera
How to work around it?
They give a year garantee on their stuff and everything is checked before it is sent to the buyer. No idea what they checked with this camera. Maybe that it WAS a camera?
I bought this camera after my adventure with the D750 that kept to get very dirty even if you did not change the lenses. It seemed to come outof nowhere. And then now this one.... supposedly to be completely in order.
Later I bought a brandnew D780. And yes, after a few months the same problems started to arise. I (with my heart in my throat) cleaned the sensor with swaps and yes... I got rid of a lot of dust, but other particles had dicided to keep residence on the sensor. I was considering to buy a bridge camera that had does not have interchangeble lenses to get rid of the dustproblem. But then my brain had a bright moment.
If we need to put the aperture to f22 to check for dirt because you do not see it otherwise..... would there not be the solution? Do NOT close to f22. I changed from shutterspeed choice (where the camera decides on the f-stop) to aperture choice, where I did not close the lens further than f11. In most cases with 1/8000 as a maximum that will do. Problem solved.
Of course it will not mean you never have to clean the sensor again. But when you do and some particles stay behind it is not a problem.
I use my camera now with much more pleasure than before!
Here two more pictures. They are crops of the left down corner of a photo. The first one taken with f22 and the second one with f11. The sensor had not been cleaned in between. I admit that on the second one you can see two vague spots, but those don't bother me. The ones on the first picture do!
KISS
Keep It Simple Stupid
The camera has a sensor cleaning option. It lets the sensor vibrate when the camera is turned on or off, or even bij both. I always had that on. But one time I thought about holding the camera DOWN when switching it on and off for maybe the dust might fall down then if it comes lose from the sensor. And it works! For months I have had no problems anymore with visible dust on my photos!! It is so logical, but till now I never came across it!