In the last month I've noticed in still pics the sky has several circles that are pretty faint. More noticeable when I edit and darken the sky/clouds. I tried with and with out nd filters. I've cleaned the camera lens and nd glass. Camera has no visible imperfections on it and always has a nd filter on to further protect lens. Take a look at attached pic to give your suggestions.
It can be enough to move the drone between 2 very different ambient temperatures for moist to form on surfaces ... for instance carry the drone between cold outside temps inside to warm temps ... or from an air conditioned environment to a warm & moist outside environment.
If it's inside the lens or on the sensor it's mainly nothing you can do ... if you open anything up you instead will get in dust particles which will be equally bad. If you're lucky it's actually moist & not stains ... so you could try to bag it together with silica gel pads & hope that will draw out any moisture.
Can be due to that the same resolutions aren't used when you film & taking stills ... which can make it more difficult to see. If you for instance have dust particles on the sensor on a DSLR camera they usually show up best if you stop down the lens ... wide open & the spots doesn't show up.
I had exactly the same issue with my Mavic Pro. For the sake of handiness I had been keeping the Mavic in the boot of my car in case I needed it. I also kept a UV filter on my lens all the time, in theory to protect it, but it seems to have had the opposite effect. When the weather got colder condensation developed inside the lens on the sensor and every photo had those little rings on it. Unfortunately the only solution was to change the camera, though it wasn't too expensive and the work was carried out by a local DJI approved dealer. I no longer store my drones in the car.
Have you tried cleaning the outside of the outer glass/lens. I think this is not removable but could be mistaken, if I am mistaken clean both its surfaces.
I would suggest that using rice is of little use. If there is 'wet' moisture present, rather than dried out water marks, a proper, bagged deliquescent would be better.
Besides loose grains of rice or deliquescent may get where they shouldn't,
Never use rice, that's more of an old wives tale. If you got the spots due to condensation drying out on the inside somewhere, then that will always be there until that dried spot has been cleaned. If you only see it in stills, it may be because the moving video is distracting your eye and it still may be there.
With that said, at least it is easy to remove from a still image in any editing software, so that's the good part!
