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How to clean a lens?
Aw, when my lenses get dusty I just throw 'em away.
Seriously, a funny aside. When I was a kid, a (now famous) photographer showed me how to clean a lens by first taking a swig of whiskey, then 'hush' a breath to fog the surface before wiping it. Really, I think he was just trying to freak me out. When I tried it on my (plastic) lens, I got sick ... it gets worse, so I'll stop here.
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How to clean a lens?
Thanks to all of you for your advice.
The most important thing to know was, if any cleaning liquid should be used or not.
I tried again and it looks better now. There are still wipe marks, but they are hardly visible. So I better leave it like it is now.
(A special "Thank you" to John. After the third glass of wodka I had a brilliant idea: I just took another glass of wodka and dipped the lens inside. I cannot see any wipe marks any longer.)
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How to clean a lens?
Thomas Tomsey's camera repair books discuss how he cleans lenses - he uses Windex and Kleenex. So perhaps we are all being too anal.
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How to clean a lens?
Buy yourself a packet of Kodak lens tissues (other kinds tend to be damaging) and a bottle of Formula MC lens cleaner. Take out two tissues (per lens) and tear one of these in half. Tear off a fresh end from the half tissue and fold this over and roll it to make a little brush, using the torn edges on the lens. Brush gently, from the middle to the edges, and be very careful when working around the edge of the lens, an area which can tend to trap dirt. Dispose of this piece. Next, fold the other half - tissue a couple of times and moisten with a drop or two of the MC, and gently wipe the lens, in a circular motion, starting in the middle and working towards the edges, stopping just short of the edges to avoid having any liquid seeping into the mount. Now, fold the other whole tissue in half, and use this to polish the lens, again working from the center towards the edges. The thing about formula MC is that it will take a bit of gentle polishing to remove all the residue, but its worth it as MC is self-protective in use. You'll need to "unfold" the tissue and refold so you can use the inner surfaces for a final polish - and should inspect the glass surfaces under an oblique light source to make sure they're truly clean.
OK - lest anyone think this is overly obsessive - it only takes a minute and is only really necessary in the case of fingerprints or other oily smudges, and of course salt spray. Don't sweat about dust - unless there's lots of it. I do recommend lens tissue over microfiber, for the simple reason that the tissue is thrown out after using it. A microfiber is designed to pull in dirt - great when its absolutely clean but increasingly risky with each use. I usually do keep a clean microfiber handy - but usually use this to clean viewfinders, loupes, and ground glass surfaces.
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How to clean a lens?
Methanol is what the manufacturers use. It will not damage any coatings, except clean the ones deposited by other cleaning fluids; call the manufacturer of your lens and ask them.
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How to clean a lens?
At work I have a $150,000 pulsed Ti:sapphire laser and have to regularly clean its mirrors. The service engineer showed me how: use Kodak lens tissue, folded and clamped in a hemostat without touching the part of the tissue that will do the cleaning, and moisten it with a drop or two of reagent-grade methanol. Swipe the mirror one time with the methanol-moistened wipe. i usually end up using lots of the tissue. I use the same method for my LF lens and it works beautifully. As mentioned in the responses above, be sure not to do this if there is debris on the lens or that will grind the coating.