Dust can usually be blown off, but when I recently bought some filters from a supplier of used equipment, one of them was particularly greasy and grubby (the filter - not the supplier!) so I asked him to clean it. He used a fluid, from an unmarked bottle and a soft cloth, but was extremely reluctant to tell me what the liquid was. <p> Presumably glass filters could be washed with detergent and water to remove grease (eg finger prints and other grime). I guess it wouldn't be safe to use a liquid or solvent on a lens, in case it damaged the special anti-reflective vapour deposited coating. <p> What is the recommended method for cleaning optics?