To thoroughly clean lenses remove dust and particles by first blowing with an upright held can of air duster or a bulb-type puffer then lightly brush the lens free of remaining dust and particles with a lens duster brush. Dust and other particles will cause the majority of scratches that diminish optical quality. Add enough drops of distilled water to wet a lint-free or microfiber cloth and hold the wet cloth to the entire lens surface for a couple of minutes. This will loosen water-soluable materials. Use a dry portion of the lint-free cotton or microfiber cloth to gently wipe the lens dry with a circular motion. If the lens still has spots or looks stained then apply at least 90% pure isopropyl alchohol to wet the cloth held against the lens for a couple of minutes and then wipe as above. Albuminous material such as blood or glycerin on the lens may require using a weak ammonia solution (one dropper of ammonia in a 1/2 cup of distilled water) as the cleaning agent, use as above. It is generally recommended to not wipe dry lenses without some kind of solvent first wetting them, always use a solvent as above. Lens pens can be used as the tip is impregnated with a cleaning compound.
Get lens covers, keep them on when the optics are not in use and surpress the urge to try to clean lenses in the field without the proper tools. A lens pen is the bare minimum of equipment in the field. Again, try to remove all dust and particles by lightly brushing and then lightly use the cleaning tip in a circular motion.
