Lens coating test

[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.

[ridax]

Back to the spots.... Those aren't deep enough to change a lens element shape so they can't influence the lens aberrations and its overall sharpness (actually, even the real scratches do not). Its only the overall contrast that can possibly be effected (due to flare). The influence of any spots is proportional to the ratio of the area occupied by all the spots together to the area of ALL the glass-to-air surfaces of the lens combined. Just do a very rough math to get the idea what a tiny fraction of a per cent that influence may ever be. And if those spots are in a multy-coated lens, most probably they are only in one outer layer of the coating. In this case, divide the ratio above by the number of layers in the coating (for example, for a 7-layer Pentax SMC or Zeiss T*, divide it by 7).

That's why I feel absolutely safe to bet you never see any difference not only in actual photos but in any meticulous tests either.

My own 80mm CZJ MC-Biometar with a lot of tiny spots in the outer coating layer, quite unsightly but showing no difference in actual performance to a mint one (two pictures in different light):

[ridax]

And if anybody still wants to conduct their own lens flare tests, I can recommend a reliable method with a calculatable outcome that does not requre any gear a hobbist film photographer doesn't have at home.

Mount your lens on an enlarger (or on a camera used in the enlarger mode, set for projection). Use a bright light source. Make the negative carrier illuminated evenly with diffused light (with a condenser enlarger, a paper diffusor put under the condensor works fine). Put a clear piece of glass in the negative carrier. Attach a small light-tight black spot in the center of the glass at the lens side. Make sure the spot is as black as it can get and is reflection-free (a piece of black velvet or some cotton wool dyed black works).

Darken the room and project the image with the lens tested. Put an exposure meter at the image plane and compare the meter readings at the large bright white outer field to its readings at the small center black spot. The difference in the meter readings is the maximum brightness range you can get at the film plane with this lens under extreme conditions (like taking a picture of a small piece of charcoal put on a field of fresh and clean snow). If your negative carrier (the bright field) is equal in size to the format you use the lens for, then the measuring conditions correspond to the lens on your camera with a perfect (big enough and properly adjusted) lens hood.

Compare the measured results for different lenses at different apertures. Compare the results for the lenses you normally consider clean enough for practical usage to the results for the same lenses thoroughly cleaned just before measuring their flare. Arrange a larger-than-the-normal-format bright field in the projection system to get the idea how much flare is added without a good lens hood....

(The exposure meter should read only the light projected on its cell, not all the light reflected back and forth inside the room. Use incident mode with a flat diffuser or better yet, without any diffuser at all with a hand-held meter. With an SLR TTL meter, use the camera without a lens. Make sure to point the meter cell directly to the lens being tested and shield it from any sideways light.)

[ridax]

And just one last remark for those who cares about their lenses' surface spots too much:

With modern lenses, both multycoated and single coated, the image brightness range limited by the lens flare often exceeds the density range that a good-looking (not flattened to much by a grade 000 paper) print is capable to reproduce. In this case adding more flare is the way to get more shadow detail in a harsh-lighted scene.

[Kevin Crisp]

I've had one lens, a 240mm Nikkor, that came to me with a spotted front element. Nothing would clean them off. Then I tried a lens pen and they came right off. I supposed it depends if there is something on top of the coating, or whether there is damage to the coating and places where it has chemically come off.

[Two23]

Ether can be purchased from wet plate suppliers such as Bostick & Sullivan, UVphotographics. Do NOT open the bottle if it's warm. Ether will boil at 95 F. I keep mine in a small dorm sized refrigerator.




Kent in SD

[John Layton]

Set up your camera with this lens on a sunny day...preferably a close distance to a woodsy area with dark shadows - and position your camera such that it is facing the wooded area, and the sunlight is falling obliquely across the front of the lens, and across those spots.

Now, take a photo without shading your lens. Next, take the same photo but with the lens shaded. Note the difference in results.

But to make this test meaningful, you really need some means of establishing a performance "benchmark" for the lens in question. Thus, in a perfect world, you would, after the above test, repeat the same sequence with an identical lens, but one without the spots.

Next best thing would to mount another lens in better condition which features similar coating technology, hopefully of about equal focal length, and repeat the test.

Edit: As for the properties of lens flare which allow it to "enhance" shadow detail - there are two sides to this.

While more flare might indeed offer the impression of more shadow detail in a negative, those shadows can often seem very flat.

In my own practice, a much better means to obtain good shadow detail (assuming I desire good contrast within those shadows) is by metering those shadows and "protecting" them with enough exposure, then dialing down my developing time (or employing a different time/agitation method) to protect highlights. Furthermore this offers me a broader "palette" with which to tailor my results in the darkroom.

But do note that the above passage was first person, and that you might want something different.