I took a few shots of the clear blue sky and filled up the frame with the light blue. To my surprise, I saw some round dots appearing at the same positions in all the four pictures I took.
They are round and the size is about 1.5mm on a picture size of 20x13cm.
I increased the contrast of the pictures and enlarged the picture size to 200% and found many more dots.
This is how they look
Are they dirts or fungus and where are they, on the lens, the mirror or the sensor?
How to get rids of them?