The best way to keep lenses and filters clean is not to get them get dirty in the first
place. This may sound trite, but every cleaning carries with it a very small risk of
causing problems. 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) is
rubbed across the lens. Of course lens cleaning tissues, cloths, brushes and fluids aren't
harder 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 really
matter what the numbers mean, but it is important to know that a material with a higher
number will scratch a material with a lower number. Here are some typical values:
| Material |
Hardness |
| BK7 glass (typical optical glass) |
610 |
| SF11 (optical glass) |
450 |
| Fluorite (CaF2) |
163 |
| MgF2 (typical lens coating) |
415 |
| Feldspars (e.g. granite) |
560 |
| Quartz (e.g. sand) |
820 |
| 316 stainless steel |
170 |
As you can see, typical coatings and most optical glasses have a hardness in the
400-600 range. This is pretty hard--harder than many metals. Try scratching a
microscope slide with a pen knife. You won't be able to do it (don't try this with
your lens, just in case). However Fluorite (a crystalline lens material used by Canon
in their "L" series telephoto lenses) is quite soft, and that's at least one
reason 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 have
lower hardness values, but again they are used as internal elements in lens designs, where
they are protected from normal cleaning procedures.
While many metals (e.g. brass, aluminum, copper, 316 stainless steel) are softer than
optical 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 minerals
such as Feldspars (e.g. Granite)--which make up most of the earth's crust--are
also 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 or
filter. The particles can be too small to see, but they can still do damage.
Cleaning Optics
The first step in cleaning an optical surface is to gently remove the surface dust. The
best way to start is by blowing off any loose dust using a good blower bulb. I'd avoid the use
of compressed gasses, which can spit small amounts of liquid propellant or cool the glass
and cause condensation. If you still see some
residual dust you can try to remove it using a soft lens brush. Hopefully these procedures
will remove any particles of hard mineral dust from the surface without causing any
damage.
While blowing or brushing can remove surface dust, it won't remove oil or material
stuck to the lens, such as the residue of sea spray (i.e. salt) left on the lens after the
water evaporates. To remove this, a solvent is often required, either water-based (to remove water-soluble compounds) or something which also dissolves oil, such as
an alcohol.
Solvents should always be applied on some sort of tissue, never poured onto a
lens. 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 the
preferred solvent since it will dissolve both oils and salts, but it can be toxic if
misused and it may be difficult to find pure. Ethanol (ordinary "alcohol") and
Iso-propanol are much less toxic and easier to find.
If you use an alcohol, make sure it's
pure 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 your
breath will condense on a cooler surface as pure water. You can then wipe the condensation
off the surface with a lens tissue or microfiber cloth.
Easy Does It
Always gently wipe the glass with the moistened tissue. Applying too much
pressure or "scrubbing" the surface is not recommended. The
solvent 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 increase
the probability of scratches.
One solvent to avoid (or to use only with the greatest care) is Acetone. It's very, very
good 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 you
have 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't
know), then acetone might be the only thing that will remove it.
Also, avoid household glass cleaners (such as Windex), which may contain ammonia
and dyes. If you want to be safe, stick with the commercially available cleaning fluids
designed specifically for use on photographic lenses.
Note that acetone and alcohols are both flamable and not good to breathe, so if you do
use them do so in a well ventilated area with no flames or ignition sources around. Avoid
skin 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 cloths
should 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 a
very, very small fibers made of a polyester/polyamide material. The fibers are often as
small as 1 micron in diameter--which is 1/100th of the diameter of a human hair. Pretty
small!
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 be
useful for cleaning optics when out in the field. There are also a number of devices
called lens pens which are also convenient to carry. They vary in design
but basically have a small pad of microfiber cloth on one end and sometimes a small
brush 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 and
filters than uncoated or monocoated versions. Fact:
Oil and grease are much more visible on multicoated optics, so it's more difficult to
remove every last visible trace. For example, a grease smear (possibly left over from a
fingerprint) which shows up on a multicoated filter would be invisible on an uncoated
filter. There would be the same amount of contamination on both. The oil is more visible
on the multicoated filter because it negates the anti-reflection effect of the coating and
so appears as a brighter spot. On an uncoated filter the surface reflectivity is
essentially unaffected, so it's much harder to see.
Myth: You need to take more care cleaning modern coated and
multicoated optics than older uncoated optics. Fact: If a
coating is properly applied, it can be almost as hard as the glass itself and it bonds
very strongly to the glass surface at the molecular level. Properly applied coatings can't
be 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 modern
name-brand multicoated lenses and filters, normal care is all that's needed
for cleaning.
Fact: If you're cleaning older lenses, extra care may be needed. Some coated lenses manufactured though the
1950s (and maybe even as late as the 1960s) had fairly soft coatings. The early Leitz 50/2
Summicron is sometimes cited as an example of a lens with a coating that can be easily
scratched. 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, so
that'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 polish
it "just to make sure". If it doesn't look dirty, leave it alone!
© Adorama Camera
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