(M4/3, Oly M10 II)
I accidentally left my ISO locked at 640, and my camera did the best it could to hide my incompetence from me. For some daylight shots, it was in the f11 range. While looking back at my pictures, I saw some weird blobs in the images. After much searching, I identified:
Same artifacts happen with different lenses. It is the camera, not the lens.
Not really visible at all on wider fstop, even 5.6 is basically invisible.
Easily seen at f11, completely image-ruining at f22
Looks like sensor dust or dirtiness. Even if it is invisible to the naked eye, it can be there. You can try to buy a few cheap sensor swabs and clean it, should do the trick
There has been some information around – possibly floated by Olympus for the purpose of securing business for their repair shops – that the sensors have some kind of coating that may be weakened/damaged by DIY cleaning.
It isn’t totally implausible, though, as its likely that Olympus’ ultrasonic sensor cleaning system has some relation to the slipperiness of the sensor surface.
I clean after each day of shooting using a loupe to see dust on sensor, then using a blower and then if still required a soft brush electrified by the blower. So far no need for more intrusive swab cleaning.
Checking for spots on the sensor is the main use I have for my pinhole lens. This thing acts as a f/256 aperture and exacerbates any sensor dust. As you can see below (crop but 100% scale) the only thing clean and sharp with these lenses is the sensor dust.
