The coating on the lens can be damaged
by excessive cleaning fluids and rubbing. There are a number
of lens cleaning fluids on the market. Kodak makes a good
one. Hoya, the filter manufacturer, markets one that I have
used in the past. You can have your camera store recommend
one. Several years ago, after hearing numerous recommendations
and remonstrances relating to countless lens cleaning products,
I spoke to the repair people at Nikon to find out which one
of them the factory preferred. Some used methyl alcohol and
some used Freon; a few used a mixture of both. Freon is, of
course, not suggested due to its negative environmental impact
and is certainly no longer used at Nikon. The pint is that
the factory uses no commercial lens cleaning product when
repairing and assembling lenses. I also spoke to the repair
people at Hasselblad (renowned for the superior quality of
their lenses) and was surprised to hear that they generally
use no solvent cleaners at all unless specifically called
for. They suggest that an exhaled breath on the glass and
a gentle removal of the mist that forms with the lens tissue
or cloth will not yield the best results and will not damage
the coating. Absence of the mist in a spot indicates contamination.
If the lens does not evenly fog after the procedure is repeated,
lens cleaning solvent is indicated.
When lens cleaning fluid is called for, place a couple of
drops on a piece of lens tissue and gently clean the glass
surface. Do not drop the liquid directly on the lens as it
can flow inside between the elements and create a real problem.
Use a dry piece of lens tissue to finish the cleaning. You
might ask your camera store to suggest a lens paper or cloth
for your lenses. There are several food ones that will significantly
scratch most coatings or leave lint, although virtually all
papers and cloths presently marketed can cause microscopic
abrasion to the lens coating which will produce no measurable
image degradation. Clean surgical-grade cotton is probably
the least abrasive material you can use. Perhaps a couple
of puffs with a blower will remove loose dust particles, leaving
a clean surface and eliminating the need for any of the above
procedures. Most important is preventing contamination of
your lens cleaning paper or cloth with gritty particles, which
will gouge the delicate lens coating.
May I also suggest that you always clean your lens before
lubricating your O-rings. Unless you wash the silicon grease
off your hands with detergent, you will transfer tiny amounts
to your lens cleaning paper. The process goes something like
this: "Couple dabs of grease on the O-ring, slide it
between my fingers gently without stretching it—ahh,
nice coat of grease on the O-ring (and on my fingers). Put
that O-ring back in the nice clean groove. Wipe off my hands.
Good, now let’s get some lens paper out of the pack.
Damn—the whole pack came out! Arrange them neatly and
stuff them back in the envelope (thereby randomly contaminating
much of the paper with grease, making the whole pack unsafe
to use). Now, since I know that if I don’t fold the
paper, my finger will go right through it and smudge the lens,
I’ll carefully fold it (practically saturating the paper
with grease) and ‘clean’ the lens. Hmmmmm, still
not clean. I’ll just repeat this procedure until I’ve
pulled the rest of my hair out."
Maybe this is why your lens is extra hard to clean sometimes?!