In case there's some little finicky stuff that's kinda stuck on the rear element of the lens that's recessed, what can be done? Tried using my blower from a variety of angles, but it doesn't seem to get the offending item off. any recommendations? or just live with the little stubborn speck? kinda worried that if the speck is actually some form of fluid, it might react with the surface coating of the rear element (if any) and screw up the lens in the long run. suggestions?