I was clicking away at the rainbow the other night and noticed that on one shot there was a rash of what appeared to be sensor dust:
I took the next one and no dust:
The difference is that the second one has been processed as I liked it etc ....but originally no dust while the first pic is splattered. The only differences between them
dirty: f22 1/80
clean: f/4 1/125
Would closing the lens to f/22 somehow do this?