Lens coating test

[j.e.simmons]

I have a lens that has “spots” in the front coating. Cleaning does not remove them. I want to test to see if they cause problems in the image, so what should I photograph for the tests? Clear open sky, an evenly lighted smooth wall? I shoot mostly landscapes so the lens will be used at longer distances.

[Kevin Crisp]

Generally this is not a problem. Try a relatively dim room interior, expose for the interior, but have an open door or a window with much brighter light on the outside. Unless you have the same lens with no coating spots, it may be hard to interpret the results since you have no control. When evaluating "cleaning marks" I can usually look at a bright distant light through the lens. If the light looks hazy or fuzzy they are significant. There will be some reduction in contrast using the lens.

Or, keep it simple. Go photograph a landscape under typical conditions and see if you like the result. Throw in some bright highlights.

[ridax]

I have a lens that has “spots” in the front coating. Cleaning does not remove them. I want to test to see if they cause problems in the image, so what should I photograph for the tests?

You shouldn't photograph anything for that type of test. The influence of those spots is so negligible that it is not detectable by any modern precision measuring instruments - to say nothing of an actual picture taken by the lens.

Even if you compare two identical lenses with and without the spots, the difference between them would depend on the degree of their glass cleanness, not on the spots you describe. (And you'll have to spend about an hour cleaning EACH glass surface with ether before you start testing the pair of lenses by the way.... but no you will not see any difference still.)

[maltfalc]

nighttime street photography. if street lights, cars and shop windows don't cause any problems, nothing will. take at least one shot with some out of focus small lights to see if the spots are visible or not.

[Alan Klein]

I have a mark on the back of my LF lens, a 75mm. How would I check out how it affects the photos?

[Bob Salomon]

I have a mark on the back of my LF lens, a 75mm. How would I check out how it affects the photos?

Use it and see.

[Alan Klein]

Use it and see.

Well I haven't noticed any bad results. But maybe I have to try it on a blank wall or maybe a black wall or whatever to bring out the defects. Is there a procedure to do that?

[Alan Klein]

Have you tried ROR? It cleans stuff that normal lens cleaner won;t clean.
https://www.bhphotovideo.com/c/produ...saAr1QEALw_wcB

[j.e.simmons]

The MSDS of the recommended ROR says it includes alcohol and ammonia. I’ve read warnings against using both of those chemicals. Can this really be safely used?

[ridax]

The MSDS of the recommended ROR says it includes alcohol and ammonia. I’ve read warnings against using both of those chemicals. Can this really be safely used?

The more powerful the solvent, the less times you need to wipe your lens surfaces. The more the wiping, the more the trouble and the more the risk of leaving cleaning marks.

Ether is the best solvent for lens cleaning (add about 10% of ethanol to slow down the ether evaporation. For pure ether, it's too fast). When I made a lot of formal lens tests, that was the only solvent I used for the glass cleaning. But ether is a medical narcotic so it's not easy to obtain in the wast majority of the world. So for practical usage, I clean my lenses with acetone.

Acetone is the second best solvent for the task that I know of - a great solvent that is pretty cheap and usually easy to buy.

Ethanol (the 96° common alcohol) is usable but much less effective then acetone. With ethanol, you have to make much more efforts rubbing your lens and still have quite a bit more dirt left on the surfaces after cleaning.

That's why those people add ammonia to their ethanol-based solvent. Being an alkali, ammonia engages into a chemical reaction with the oily substances that alcohol can't dissolve fast enough. Oil + alkali = soap, and soap is far easily soluble then the original oil. But beware alkalies can dissolve the glass itself (though that's a very slow process). Better use acetone.

Why they sell a stuff like this? Because their goal is to offer a solvent that's usable for cleaning a lot of surfaces, not only the glass ones. A lot of plastics are readily soluble in ether and in acetone. Those include some of my own plastic-lens reading glasses (not all of them; I've actually ruined a pair but regularly clean the rest of my batch with acetone), the Canon EOS SLR eyepiece lenses, and so on. Besides, ether and acetone dissolve the vanish that's used in older generations of lenses to blacken the barrels' and shutters' metal parts. Unless you lens in modern, avoid touching its black metal parts with acetone. It also dissolves the Canada balsam that's used in the older lenses as an optical cement. Don't splash a pool of acetone on your lens to let it go inside right to the edges of the cemented elements....

And - never touch the cotton wool you use for lens cleaning with your bare hands; human skin may have more oily stuff on it than the glass you want to clean.