This guide to cleaning coated optical glass of photographic lenses and filters is designed for safety and like-new optical clarity. If you’re not a perfectionist you can stop part way through.

Few used lenses & filters sold on eBay are clean. Even new / old-stock filters require cleaning to remove haze caused by packaging materials. While most lenses described as EX+ or better will clean up without revealing significant flaws, the odds are somewhat lower with filters. Hopefully I can encourage a few more sellers to try cleaning, which is required to access condition.

If you follow all the precautions outlined, it’s very unlikely that you could cause any damage. The recommended solvents can have an adverse effect on plastics. If you have a plastic lens or filter like Cokin, the basic technique can be applied, but you’ll need to stick with safer, albeit less effective cleaning solutions.

It may seem a little complex, but after reading the article, you'll have enough info to make an informed decision about any short-cuts you might want to take. If you don't do much photography, & don't plan to do much lens cleaning, you could just get yourself a lens cloth (available on eBay). Make sure there's no dust on the filter, then breathe vapor on it before wiping with the cloth. That should work fine for a single fingerprint or smudge, but won’t work on a lens or filter on which any sort of build-up has been allowed to accumulate.

Procedure

Most of the cleaning technique is the same for both lenses & filters, with the following exception: It’s OK to apply solution directly to a filter, as it facilitates cleaning right to the edge. However, when cleaning a lens, solution should only be applied to the tissue to prevent it from getting inside the lens.

1. First you need to remove all dust & grit. This is a crucial step because these particles can scratch glass. They present nearly all the risk involved in cleaning your lens or filter. Optical glass is hard & even the coatings are quite hard, but those tiny specks can be harder substances like minerals, silica or rock dust. I use regulated compressed nitrogen for this, but if you don’t want to deal with a 100 lb. tank, you can use an inexpensive blower-bush. Ordinary canned air is risky because of it’s potential to spray freezing liquid that can leave a permanent mark. And, blowing by mouth doesn’t work very well because the resulting condensation tends to hold small particles in place.

Compressed nitrogen tank w/ blower brush sitting on of top the gauges. There are some new blower designs available, but don’t use a leaf blower.
 
2. It’s grit that makes reusable cleaning devices unsafe & that’s why I recommend old fashioned lens tissue. Kodak makes the best, & Olympus is almost as good. The packs are kind of small, but probably conveniently so for most people. I prefer the larger Kimwipes that come 280 in a box. Avoid silicon treated tissues intended for eyeglasses. Start by folding the tissue so that you can still tear it, then tear 1 edge & roll the folded tissue so that the torn edge becomes a soft end. You may be tempted to start w/o tearing, but the 1st attempt is when your glass is most vulnerable to residual grit. Particles too small to see can still damage your lens. The advantage of the camel hair blower-brush & the torn tissue is that they won’t press grit into your glass, so tear for maximum softness.

Kodak has 50; Olympus 100, & Kimwipes 280 sheets per pack. Torn rolled tissue with lens chamois. Leather chamois is a little more effective on clean glass, but microfiber is probably safer.
 
3. I have a lot of little bottles labeled “lens cleaner” & none of them do a respectable job, however they are safer to use with plastics. You’ll want to keep alcohol based solutions away from plastics. (I tested several different types of plastic w/o any adverse effect, but when attempting to clean a Cokin filter, some color transferred to the tissue, which can’t be good). The preferred cleaning solution is Eclipse, with some cheaper substitutes available (see tech section below). Ordinary lens cleaners leave a lot of residue, but Eclipse leaves none. Photographic Solutions also makes Pec Pads for use with Eclipse. They work quite well, & are soft w/o the need for tearing, but can't be torn to expose a new cleaning surface like tissue, & I have not been able to do as well with them. (Note: the manufacturer has assured me of the safety of their products with coated lenses).

Eclipse by Photographic Solutions. 1 quart SLX vs. 2 oz. Eclipse, $8 each.
 
4. Place a few drops on the filter surface (don't do this with a lens, or solution may get inside) & swirl it around with the soft, torn end of your rolled tissue, paying special attention to the edge of the glass. Frequently unroll the tissue & tear a new edge. While Eclipse itself leaves no residue, material dissolved in it will, so you have to apply it repeatedly to achieve perfection. If your not that concerned with perfection, you can stop here.
 
5. Inspection should be made with a very small, bright light source (& a dark background with filters). When the glass appears quite clean, finish the job with a soft clean micro-fiber lens cloth. Note that the cloths don't work for initial cleaning, & require frequent washing to work at all. Hand-wash w/ dishwashing liquid. A used cloth can transfer oil from finger tips to the lens or filter. If there’s residual oil on the filter, a lens cloth will just move the oil around, so you have to go back to the eclipse & tissue. If there’s grit on the cloth, you’ve got a bigger problem. (The cloth is used to speed the process. It might be safer not to use it, but you could end up swirling tissue for a long time).

Technical Notes

Eclipse is also the recommended solution for cleaning digital camera sensors. It’s highly refined methanol, significantly purer than medical grade methanol. You could use cheaper, less refined methanol - CAS # 67-56-1 for filters, but not on sensors. I generally reserve Eclipse for lenses or sensors, & use safer, less aggressive SLX denatured alcohol for cleaning filters in quantity (hardware, paint store; Home Depot). This is ethanol that has been denatured with methanol so that we can’t drink it (don’t smoke the rolled up lens tissue either).

I try to avoid frequent cleaning of my personal lenses by keeping UV filters on them. This can be a little inconvenient because of the need to remove the UV when using other filters, but I think it’s worthwhile.

Coated optics that have been stored dirty for a long period of time may have fungus spots on the surface. A combination of organic “dirt” & atmospheric moisture is a conducive environment for fungus, which can be removed, but it secretes an acid that permanently etches the optical coating. These can’t be restored, but regular cleaning (once or twice a year) will prevent this from happening.