Although I declare myself very satisfied with my Nikon Coolpix P900, I've had in my recent trip in South America numerous pictures blurred by a random artifact, and I would like to know how to avoid that in future.
The problem occurred always at high altitude (between 3000 and 5000 meters above sea level), in full daylight (very high EV, high percentage of blue in the colour temperature), in the upper zone of the optical zoom (between 1,000 and 2,000 mm of 35-mm-equivalent zoom factor).
The artifact is a random spot of blurred pixels just in the middle of correctly focused ones. Sort of an artistic post-production filter of the "Impressionist painting" style. Something like you have just done a watercolor painting and you gently poke with the point of your fingers on the fresh paint, leaving some of it untouched. Where you touch, the image gets blurred.
Three examples are available here.
I was working with Auto White Balance, mostly shooting with P (Program) mode, in max-dimension JPG format (the P900 does not, unfortunately, allow RAW pictures) and Noise Reduction factor on Normal (where + and - are also available).
Most of the other images, shot at the same extreme distances, are perfectly sharp. When they were out of focus, the image was just faded, but not full of artifacts like these.
Any idea on the origin of the problem, and how to avoid it?
Setting the noise reduction on "-" rather than on "Normal"?