I have a STRONG Xenon flashlight; when I use this to examine my lenses (Leica and Hassy) I NEVER see perfectly clear glass. Things which one cannot see with the "naked" eye even using the Sun, show up on/in the glass. Things that pop up are:

Speckles (happens in my old car windshield too)like small bright tinsel

Fine Pits

SLIGHT internal fog/smoke, especially when the flash light is held at diff. angles.

In fact, I've NEVER seen a crystal clear lens, even new ones. The lenses perform fine, but it's buyer beware WRT "mint" lenses. I guess nothing's perfect.

OK Leica Users, break out the flashlights and report what you see. I keep my lenses scrupiously clean and use filters. I only use water vapor for cleaning.

Does glass/coating naturally deteriorate/corrode?

Does it have anything to do with radioactive glass?

On a separate but related topic: water vapor on a "perfect" lens surface appears and evaporates uniformly. On occasion, the condensed vapor appears "splotchy". I guess the coating has undergone some unreversible/uncleanible compositional change. How would this affect the light wavelength passing through this?

-- chris chen (chrischen@msn.com), March 06, 2002

Answers

Until starting on this forum I never heard or thought of the flashlight test, being happy with looking through them at an ordinary light source. Once I tried it I was horrified. Of more than a dozen lenses (even an almost new 50mm Summicron) only two were free of "smoke," a 35mm Summicron-M, and 35mm Summilux-M RF. Of course they all make perfectly fine pictures, even the old coated Summar. None of my Zeiss, Schneider or Japanese lenses show any such problems. I don't know what it means.

-- Bill (bmitch@comcast.net), March 06, 2002.

i have not checked my lenses, but i must say that i make it my policy NEVER to clean the surfaces of my lenses with anything but compressed air (and then only very occasionally). when i buy a new lens, i slap a filter on. i then use a little compressed air from time to time to clean dust that gets on the surface when i change filters. i really believe there is absolutely no safe way to clean a lens.

-- roger michel (michel@tcn.org), March 06, 2002.

As an academic exercise this is fascinating. But what could you possibly be taking pictures of (and how) where any of this would make even the slightest difference?

-- Keith Davis (leica4ever@yahoo.com), March 06, 2002.

I'm to paranoid to check my lenses. I did have DAG do a CLA on a Rigid Summicron and it performed better when I got it back. I thought the lens was pristine when I sent it to him.

-- Bud (budcook@attglobal.net), March 06, 2002.

Chris

My experience is that most people obsess about all this way too much. I agree with you - the brighter the light the more dust particles and other "imperfections" you see. Just look at any new projector lens when the projector is switched on and you see the lens to be full of dust and minute particles. It matters not a jot. The more you look the more you find. Personally, I just hold it up to a regular light source (40-60W blub) and check it out. I expect to see some dust if it is a secondhand lens and even if it is new a few specks are normal. It is all a question of what is acceptable to you. I have always chosen lenses this way and have never had a problem. If you can see large scratches and or terrible haze then I pass, otherwise I reckon you can pretty well tell just by looking through the lens "naturally" using just a bright sky or any regular light source. It is a recipe for worry and heartache if one expects even a new lens to be totally dust or particle free. Still I am sure this will not stop many people worrying. Remember that none of these dust particles will be imaged any where near the film plane so their effect is insignificant.

-- Robin Smith (smith_robin@hotmail.com), March 06, 2002.

OK,

I'm a "little" obsessive; comes from being paid to be attentive to detail, as an engineer. Or, maybe it's the other way around.

I know lenses are not constructed in clean rooms and don't deserve the care the Hubble telescope got/gets. But, all things being equal, I prefer clean, this includes: home, car, bike, LP's, body, if not mind.

I guess the original comment was one: regarding the natural degradation of glass/coatings and, 2: quality control. After all, with the expense of the lenses one would think we deserve better.

-- chris chen (chrischen@msn.com), March 06, 2002.

Chris:

I noticed this same phenomenon a few months back. It seemed ALL of my lenses had crud of varying degrees on or in them. Then I cleaned the front glass of my flashlight and PRESTO! -- about 80% of the crud in my lenses disappeared! I realized that a looking through a lens backwards is just like looking through a very good loupe, and it was magnifying the crud on my flashlight. FWIW...

-- Jack Flesher (jbflesher@msn.com), March 06, 2002.

The flash light test is very useful if you know what you are looking for, and aren't just driving yourself nuts going totally anal retentive. I was sold two lenses with fungus in them that affected the image quality and will only get worse, and you would not spot this problem by just glancing through the lens. (got my money back and apologies on both the lenses). Dust is not an issue, nor are bubbles or slight coating flaws/cleaning marks/ or a few light scratches or pin size chips. I have a 50 years old East German Zeiss 50mm f1.5 T* Sonnar that has so many of these types of flaws in it, and it still rivals my current Summicron at all but the widest aperture.

That milky haze often seen in old Leica lenses can rob contrast and increases flare in backlit situations. I have had a few hazy lenses cleaned, and the results are noticeably better images especially at wide apertures.

-- Andrew Schank (aschank@flash.net), March 06, 2002.

(still laughing about cleaning the flashlight. . .)

Some of that "SLIGHT internal fog/smoke, especially when the flash light is held at diff. angles" is probably just normal lens flare and not a reflection (excuse the pun) of any deterioration of the coating.

-- Mike Dixon (mike@mikedixonphotography.com), March 06, 2002.

I must confess I examine my new lenses too much when I first buy them, hence my few returns. I don't mind a few specks of stuff here and there, but scratches and weird shiny imperfections are a no-go on a 2 or 3 thousand dollar lens. That said, when I'm satisfied with a lens initially, I clean it if need be, stick a filter on it, and keep it well protected from dust and moisture. I guess there are a lot of other factors that you won't be able to see that affect performance- element alignment etc.

-- James (snodoggydogg@hotmail.com), March 06, 2002.

" I don't know what it means."

It means you guys are spending way too much time looking for this kind of stuff instead of out using the lenses to make photographs. And that is all it means.

-- Charles (c.mason@uaf.edu), March 06, 2002.

What you see will cause a little flare and image degradation...

If you take a picture of a Xenon flashlight.

Also, keep in mind that your eye should be where the film plane will be. Spotting flare and dust by keeping your eye well off axis, beyond the perimeter of the film gate rectangle is pointless. This is light that bounces around the camera box and is absorbed by the black paint.

So the only tangentially meaningful test is to mount the lens on the camera, open up the shutter on B and use your other (or third) arm to shine the flashlight at the lens. Then look for flare, dust spots etc. Even then, whatever you spot will likely only degrade the image in extreme lighting conditions, such as shooting a light source, if at all.

All in my humble speculative opinion, of course :-)

-- Mani Sitaraman (bindumani@pacific.net.sg), March 06, 2002.

I picked up a DR Summicron, with the close-focus eyes, for $350.00 because the store felt that it had not passed the flashlight test. When I tried my flashlight on it, I could see the problems. Rather foggy and a little separation around the edge. A roll of Delta Pro 400, though, and I could tell this lens can be used as-is if desired. It's crisper than my collapsible Summicron, which has no such problems. Nevertheless, it's going to Focal Point as soon as I can find time to pack it up. I'm even tempted to have John strip it and recoat with the latest coating, just to see what it can do.

-- Bob Fleischman (RFXMAIL@prodigy.net), March 06, 2002.

Chris,

Here's a link on keeping your vinyl clean...

And what are you driving? I once had an old Saab 95 wagon that had a pitted windscreen, drove me crazy...(argh!)

-- Dave Doyle (soilsouthlessspam@cox.net), March 07, 2002.

Shine well collimated light through a lens that you are considering purchasing as a user, but I would refrain from doing this after you bring the lens home. As others have posted, you will only be anguished by the results. This is especially true of older lenses from the the 50s and 60s (and earlier, of course).

ALL lenses will develop some degree of fogging just due to age related changes in the glass. Often this can be removed by disassembly and cleaning, if it is significant. Some earlier lenses (eg., 50/2 Summars and 50/1.5 Xenons) are particularly prone to develop lens element separation, which can be interpreted as "fogging" but has a slightly different appearance. Fungus has been mentioned above, particularly when lenses are stored for long periods of time, unused, in hot humid climate.

There is this peculiar haze which lenses from the 50s and 60s are especially prone to develop, Leica afficionados have long argued back and forth about the cause(s). Some say it is due to the lubricant *whale oild) leaking out and causing the glass to cloud. Others say that over time, the glass absorbs water, which causes haziness. Your guess is as good as mine. FWIW, this particular problem is unlikely in the later lenses, with serial numbers above 3 million. Whatever it was, they seem to have taken care of it.

My advise, unless you want to get an ulcer, don't look!

-- Eliot (erosen@lij.edu), March 07, 2002.

I have a fairly used Zeiss T* 50mm 1.4 which is pretty dusty inside, but doesnt affect the image quality. Same goes for lenses with scratches, it hardly makes any difference. I would say stop worrying!

-- Karl Yik (karl.yik@dk.com), March 07, 2002.

Let's put it this way:

You pay extra for diamonds with fewer flaws in them; they don't necessarily look any different on someones hand several feet away. So, you come to find out its got more flaws than what you paid for or is a fake. Is this retentive?

-- chris chen (chrischen@msn.com), March 07, 2002.