The major problem is that each time you clean your lens, you risk scratching it. Scratching occurs when something harder than glass (or the optical coating) isrubbed across the lens. Of course lens cleaning tissues, cloths, brushes and fluids aren'tharder than glass, but dust and dirt may be.
Hard facts about lenses
Hardness can be measured on the Knoop scale. For our purposes, it doesn't reallymatter what the numbers mean, but it is important to know that a material with a highernumber will scratch a material with a lower number. Here are some typical values:
|BK7 glass (typical optical glass)
|SF11 (optical glass)
|MgF2 (typical lens coating)
|Feldspars (e.g. granite)
|Quartz (e.g. sand)
|316 stainless steel
As you can see, typical coatings and most optical glasses have a hardness in the400-600 range. This is pretty hard--harder than many metals. Try scratching amicroscope slide with a pen knife. You won't be able to do it (don't try this withyour lens, just in case). However Fluorite (a crystalline lens material used by Canonin their "L" series telephoto lenses) is quite soft, and that's at least onereason why fluorite lens elements are normally positioned between regular glass elements.
If a fluorite element was used as the front element in a lens it would easily scratch. So,if the design calls for that, a permanent multicoated flat glass "filter"element is used in front of it. There are other special glass formulations which havelower hardness values, but again they are used as internal elements in lens designs, wherethey are protected from normal cleaning procedures.
While many metals (e.g. brass, aluminum, copper, 316 stainless steel) are softer thanoptical glass and thus won't produce scratches, many minerals are harder and will. Sand(quartz) will easily scratch most optical glasses and coatings, Many silicate mineralssuch as Feldspars (e.g. Granite)--which make up most of the earth's crust--arealso hard enough to scratch a typical optical glass. A lot of "dust" and"dirt" contain bits of ground-up rock, and that's what can scratch your lens orfilter. The particles can be too small to see, but they can still do damage.
The first step in cleaning an optical surface is to gently remove the surface dust. Thebest way to start is by blowing off any loose dust using a good blower bulb. I'd avoid the useof compressed gasses, which can spit small amounts of liquid propellant or cool the glassand cause condensation. If you still see someresidual dust you can try to remove it using a soft lens brush. Hopefully these procedureswill remove any particles of hard mineral dust from the surface without causing anydamage.
While blowing or brushing can remove surface dust, it won't remove oil or materialstuck to the lens, such as the residue of sea spray (i.e. salt) left on the lens after thewater evaporates. To remove this, a solvent is often required, either water-based (to remove water-soluble compounds) or something which also dissolves oil, such asan alcohol.
Solvents should always be applied on some sort of tissue, never poured onto alens. You don't want excess liquid getting inside the lens where it can, for example,dissolve lubricating oils and redeposit them on internal elements.
You can buy lens cleaning fluids or, if you can find them,you can use pure alcohols such as methanol, ethanol or iso-propanol. Methanol is thepreferred solvent since it will dissolve both oils and salts, but it can be toxic ifmisused and it may be difficult to find pure. Ethanol (ordinary "alcohol") andIso-propanol are much less toxic and easier to find.
If you use an alcohol, make sure it'spure and doesn't have some sort of non-volatile additive. You can also try distilled water(water without any minerals dissolved in it). One way to put a film of"distilled" water on a surface is to breath on it. The water vapor in yourbreath will condense on a cooler surface as pure water. You can then wipe the condensationoff the surface with a lens tissue or microfiber cloth.
Easy Does It
Always gently wipe the glass with the moistened tissue. Applying too muchpressure or "scrubbing" the surface is not recommended. Thesolvent should dissolve any remaing material and the tissue or cloth will then soak it up.If there should be any microscopic particles of grit present, rubbing hard will increasethe probability of scratches.
One solvent to avoid (or to use only with the greatest care) is Acetone. It's very, verygood at removing grease. However it's also very, very good at dissolving plastics,adhesives and paint. It's also difficult to find commercially in a pure form. Unless youhave a surface so badly contaminated that an alcohol won't clean it, I'd avoid Acetone.However if you get something like tar on the glass (though how you'd do that, I don'tknow), then acetone might be the only thing that will remove it.
Also, avoid household glass cleaners (such as Windex), which may contain ammoniaand dyes. If you want to be safe, stick with the commercially available cleaning fluidsdesigned specifically for use on photographic lenses.
Note that acetone and alcohols are both flamable and not good to breathe, so if you douse them do so in a well ventilated area with no flames or ignition sources around. Avoidskin contact as much as possible.
Keep It Clean
A word about lens tissues and lens cloths. Make sure they are clean .Lens tissues should be used once then thrown away. The very last thing you want to be wiping across your lens is a tissue with a piece of grit embedded in it! Lens cloth sshould be washed frequently and kept in a clean plastic bag when not in use.
Microfiber cloths are excellent and the only type of cloth I would use myself. They are made of avery, very small fibers made of a polyester/polyamide material. The fibers are often assmall as 1 micron in diameter--which is 1/100th of the diameter of a human hair. Prettysmall!
The fibers are also often wedge-shaped or triangular rather than smooth and round.They act to "suck up" dirt and oil when wiped over a surface and absorb them via a mechanism which resembles capillary action between the tiny fibers. They can be so effective at grease and oil removal that you might not even need a solvent.
Zeiss has lens tissues pre-moistened with an optical cleaning solvent which can beuseful for cleaning optics when out in the field. There are also a number of devicescalled lens pens which are also convenient to carry. They vary in designbut basically have a small pad of microfiber cloth on one end and sometimes a smallbrush on the other. The brush is used to dislodge and remove dust, while the microfiber pad can be used to remove grease or oil from the surface.
Lens Cleaning Facts and Myths
Myth: It's more difficult to clean dirt from multicoated lenses andfilters than uncoated or monocoated versions. Fact:Oil and grease are much more visible on multicoated optics, so it's more difficult toremove every last visible trace. For example, a grease smear (possibly left over from afingerprint) which shows up on a multicoated filter would be invisible on an uncoatedfilter. There would be the same amount of contamination on both. The oil is more visibleon the multicoated filter because it negates the anti-reflection effect of the coating andso appears as a brighter spot. On an uncoated filter the surface reflectivity isessentially unaffected, so it's much harder to see.
Myth: You need to take more care cleaning modern coated andmulticoated optics than older uncoated optics. Fact: If acoating is properly applied, it can be almost as hard as the glass itself and it bondsvery strongly to the glass surface at the molecular level. Properly applied coatings can'tbe removed with lens cleaning solvents, nor can they be rubbed off. Of course,anything is possible if the coating hasn't been applied properly. However for most modernname-brand multicoated lenses and filters, normal care is all that's neededfor cleaning.
Fact: If you're cleaning older lenses, extra care may be needed. Some coated lenses manufactured though the1950s (and maybe even as late as the 1960s) had fairly soft coatings. The early Leitz 50/2Summicron is sometimes cited as an example of a lens with a coating that can be easilyscratched. It's also possible that some early coatings don't adhere to the glass as well as modern coatings and can also be damaged by cleaning fluids containing ammonia, sothat's another factor to bear in mind when working with coated optics made before the 1960s.
Not Sure? Don't Clean!
Just when should you clean a lens or filter? Well, the short answer is as infrequently as possible. It actually takes quite a lot of dirt on an optical surface before the image quality noticeably degrades. A surface with a lot of "cleaning marks" (i.e.light scratches) will do more damage to the image than one with a few specs of dust."Cleaning marks" tend to scatter light and so lower contrast. The best advice is to blow or brush loose dust off a lens when you see it there, but don't clean and polishit "just to make sure". If it doesn't look dirty, leave it alone!
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