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]

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

[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.

[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?

[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?

[Mark Sawyer]

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?

Anti-reflective coatings are primarily to reduce flare, so photograph in a situation where flare is a major issue, such as into the sun. Problem is, as ridax said, without a similar image from a perfect lens, you have nothing to compare the results to.

On the bright side, since the spot is on the front element, any reflected light will reflect outward and have no effect. Inside the lens, light starts bouncing around, and that's where you might see some effect. But honestly, I wouldn't expect much difference even there.