Quote Originally Posted by Oren


IAMB, how do you know it's oil? I'm sorry but I disagree, it looks like normal dust to me. Anyhow, with the given info - i.e you have no idea how the picture was taken and what kind of PP was done, you can't really tell anything for sure.
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The main reason I call some of the spots (not all of them) oil is the refraction around the borders of the spots. If you look closely, you'll see a rainbow halo around a number of the spots. It takes a fairly extraordinary set of circumstances to see haloes like the ones I'm seeing if it's just dust.





The other reason is experience. As a microbiologist, I've seen oil spots on more optics than I care to think about, and these look exactly like what I'm used to seeing with oil spots and looks nothing like what I see when I have dust on my camera sensors.





I am, however, perfectly open to the possibility of being completely wrong.