Lens Cleaning- (this is from a repair list of camera repair folks)
First of all, the best way to clean a lens is don't. Some dust on
the front of your lens is normal and will not have any affect on your
photos. Just blow it off periodically. Always point the lens down
when you blow dust off the front so it won't just settle back down
onto the lens. I use an air compressor that I bought from an
industrial surplus outfit more than 30 years ago. It has an in-line
filter to prevent any oil from being blown out the nozzle.
If you get something that air won't blow off, use a brush to try and
loosen it. A soft "camel hair" artist's brush is ideal for this.
(I've always wondered why these are sold as camel hair when they are
actually made from squirrel's tail hair!)
If you get something really stubborn that a brush won't budge, then
you have to resort to "industrial strength" efforts.
BTW, how much effect a smudge will have on your photos depends on
where it is as much as what it is, and also depends on the focal
length of the lens. A smudge toward the outer edge of a lens won't
have any effect at all if you are shooting with the lens stopped down
some since you are only using the edges of the lens when you shoot
wide open. Anything on the front of a lens toward the center will
have more effect on a wide angle and less effect on a telephoto. In
all cases a smudge on the rear of a lens will have more of an effect
than one on the front.
So if you get something stubborn, first try to determine what it is.
If it is greasy like a fingerprint you'll need something that cuts
oil to get it off. Acetone or ROR is the thing to use, on the tip of
a folded cotton cloth or on a cotton swab. It may take several
cleanings to get the grease off. Don't scrub, press lightly, and
repeat until the offending crud is gone. Then clean the whole
surface with Bonito or a similar cleaner. Bonito is not sold in the
USA, unfortunately. I always bring some home when I visit Germany.
Similar but not identical cleaners sold in the USA are Ultra Clean
(made by Novi Optical Cleaning Products, Novi, MI 48376) and Rexton
Optyl 7 (B & H and Adorama carry it). These come in spray bottles,
but don't spray them on the lens, spray them on the cotton cloth.
Generally speaking I find amateurs more obsessed with cleaning their
lenses than pro photographers who know a bit of dust on a lens does
I absolutely DO NOT recommend keeping UV or "protective" filters on
lenses. They introduce two additional air/glass interfaces which can
cause flare. Even the most expensive ones often are not optically
flat and parallel. I only use a protective filter when shooting on
the beach to avoid salt spray, in the desert when it is very windy,
or in some industrial situations. The rest of the time I want my
lenses to perform as designed.