[stevebrot]

It depends on the coating and the cleaning technique. A few examples:

• It is almost impossible to find a Canon rangefinder lens that does not have cleaning marks. Ditto for former Soviet Jupiter-9 rangefinder lenses. Most of these marks are only as deep as the coating, though I have seen a few where little of the original coating remained. These are vintage lenses with early coatings.
• I have ruined more than one filter in my youth after applying the "clean" corner of a t-shirt. Thankfully, no lenses to date.
• I have seen terrible cleaning marks on used lenses (both front and rear elements) that looked like someone had applied steel wool. Yes, I would suggest that the scratches effected image quality. Remember, it is the angle of the glass surface that determines the refraction. Make a scratch and a portion of the light goes the "wrong" way. Remember too that the coating is there to provide more efficient light transmission and less reflection.

What do I do?

• I (almost*) never do dry cleaning...no, not even a microfiber cloth
• I usually use Windex sprayed on a frayed piece of paper toweling and used as a flexible "brush" to apply the solution to the face of the lens
• I follow with a clean, soft cotton rag (scrap of old t-shirt) in a light circular motion
• I do not apply pressure or "scrub"
• If there is still stuff on the lens, I repeat the above

The main issue is mineral dust that is probably harder than the glass and/or the coatings. It can be embedded in a cleaning cloth. Not good.

Prevention?

• Use your lens caps and keep the caps themselves clean
• Don't let dirt accumulate in your camera bag
• Your lens hood is the best defense against prints from errant fingers
• Don't sneeze or cough on the camera

* Yes, almost. I do keep a "known clean" microfiber chamois in my bag and have been known to use it lightly in combination with a lot of condensed breath as a lubricant. The microfiber does a good job of wicking oils and other liquids off the lens surface.


Steve

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Last edited by stevebrot; 03-23-2014 at 08:11 PM.

[Al_Kahollick]

Here's my 2 cents, advice I received some 30-odd years ago.


DON'T: Use paper towels - too rough, use Windex / household cleaners - too harsh, also contain dyes that can / will leave a haze, dry clean a lens with ANY type of cloth - dirt gets trapped in the fibers and can scratch coatings, wipe across the lens vertically or horizontally - any scratches you do make will be more visible / effect pic quality.


DO: Blow dirt / dust with a rocket blower before any "wet" cleaning, Always wet clean - any dirt dust will be picked up by the liquid reducing the chance of scratches (same reason for not "dry" wiping bird poop off your car), Use lens tissue and cleaning formula, follow mfg. directions - they're made for cleaning lens glass without leaving behind residue or scratches, Clean your lens in a circular motion starting at the center and working out toward edge - any scratches you may make will be far less visible and have less of an impact on pic quality.


Todays coatings are far superior to what I started out with, but why tempt fate? The above "rules" have served me well over the years, and I've yet to scratch a lens by following them. My very first SLR lens looks as good today as the day I bought it.

[Just1MoreDave]

I have my brother's old *ist DL here. He wore out the original eyecup, and cleaned the viewfinder so often, all the coating is worn off. I think that might be a worst-case scenario. I've seen another DL viewfinder with scratches, but with no particular care, my DS is fine.