+1. I use lens cleaning cloths or even toilet paper, depending on the severity. If there are smear marks or fingerprints on a lens, I use pure alcohol, obtainable from some/most lab suppliers or pharmacies. It must NOT be surgical spirits, as those sometimes contain aromatic oils. If you use toilet paper (soft double-ply), it will leave some lint. Wait until the alcohol has dried, then blow or brush those off with a lens pen. If your lens pen becomes greasy, clean the bristles with alcohol. Don't use acetone or other solvents meant for paints and glues etc. One of the things that may happen is solvent finding its way into the glue between the elements of a doublet or triplet and causing separation. For internal cleaning, I take my stuff to a technician. I am not brave enough to risk doing such things myself. One reason for that is that some elements (in Nikkors at least) contain soft coatings, and attempting to clean them will ruin the lens. It is usually the back coating of the first element, but there are some lenses where other elements also have this problem.
Originally Posted by HTF III