Lens Glass Inspection

charles pappas · October 24, 2017

Could anyone offer any tips on evaluating lens glass, especially with no means to film test this lens.

I have attached six photos from an Angenieux 9.5 - 95 lens (Camflex mount I have posted about earlier). At the front glass I can see at least seven or so blemishes plus a few more around the perimeter. (There is also an extremely fine scratch about 1/2 inch long no clearly visible in photos.)

At the back element photos numerous specs are visible.

Taking the lens outside and looking through it, a dark spec or two appear to move with the image.

I know this is nebulous, and clearly the less seen on the glass the better, but any comments would be appreciated. Thank you.

Dom Jaeger · October 24, 2017

Typically, coating blemishes and fine scratches just tend to disperse light, causing veiling glare and a slight reduction in contrast. You might get some loss of fine detail as well, depending on how much of the element coating is damaged. A rear element is more crucial than a front one in that respect.

Fungus will do the same, and tends to spread, so is among the worst things to find in a lens. Often it cleans up if it hasn't been left for too long. I can't tell if your lens has fungus, it looks like fine spidery lines.

Isolated specks are usually fine unless really large, or there are too many.

Deep scratches or chips can catch and refract light, causing flares. How noticeable it is depends on the focal length and stop and lighting situation.

There could be other issues with an old zoom like this that have more impact on the image quality, such as the back-focus setting, wear to the zoom or focus mechanics, decentration of lens elements, oil on the iris blades. A film test is the only real way to check, or have a technician project the lens with a lens test projector.

charles pappas · November 25, 2017

Mr. Jaeger, or anyone, I'm not familiar with a lens test projector - can they project through a wide variety of lenses regardless of the lens mount, or is the test projector restricted to testing lenses of a particular mount or for which it has the appropriate adapter. Thank you.

Samuel Berger · November 25, 2017

Mr. Jaeger, or anyone, I'm not familiar with a lens test projector - can they project through a wide variety of lenses regardless of the lens mount, or is the test projector restricted to testing lenses of a particular mount or for which it has the appropriate adapter. Thank you.

A lens projector is a $50,000 machine used by professionals. It doesn't care about lens mount. If you have about $200-$300 to spend you can get the lens cleaned up at Bernie's. http://www.super16inc.com

If you are considering buying that lens, all that comes to mind is that if all I had to go by were these pics, I wouldn't get them; the spidery lines could be fungus and that would be bad.

Simon Wyss · November 25, 2017

IMHO you can’t tell anything about a lens from just outside. Let me explain.

I have taken apart lenses from simple two-elements to more-than-a-dozen-elements systems, and put them back together. It is only upon inspection of each single element that you can judge on something. I must see and feel the lens seat with the mount, have an overview of the diaphragm mechanics and everything. Grease and dust can be removed easily but once a barrel or a ring has really gone out of round the case is lost.

Angénieux didn’t make the worst lenses but they’re still French. That means the mechanics are in about the middle field of robustness. Younger products have anodized aluminum barrels, quite hard and corrosion resistant. Yet, once taken too hard a blow, they cannot be brought back to round. Metals bend buckling. So the very first check you should and can do on the outside is to search for binding spots between the moving parts. It takes a certain ability to discern between something sticky between threads and true mechanical problems, I have to say. If you find a clog in the diaphragm movement, alarm.

Right today I began work on a 2 inch Eymax Anastigmat for Eyemo cameras, a simple triplet made by Wollensak. I must say that I like the relatively heavy brass body and the sturdy iris blade bolts. Maybe others can comment more precisely on German zoom lenses, for me they are just rugged enough.

Dom Jaeger · November 26, 2017

Mr. Jaeger, or anyone, I'm not familiar with a lens test projector - can they project through a wide variety of lenses regardless of the lens mount, or is the test projector restricted to testing lenses of a particular mount or for which it has the appropriate adapter. Thank you.

A typical lens test projector has interchangeable mounts, mine for example came with 6 different mounts, and I've custom made more. Or you can adapt the lens itself to a more common mount, though then you can introduce errors. They are a professional tool for lens technicians and rental houses, and can cost around $20,000 new. They are essential when we work on everything from student lenses to the optics used on feature films.

I mentioned it simply because it is the best way to quickly analyse a lens, it will tell you everything from back-focus setting, to optical aberrations, to whether internal elements are fogged, to mechanical issues.

But when it comes to vintage optics like that Angenieux zoom, which you can assume will be lower in contrast and less sharp than a modern lens, I wouldn't worry too much about inspecting the glass. As I said before, fine scratches, dust, and other blemishes won't show up, they'll just make it slightly more low-con than it already is, and very few vintage lenses are without fine scratches, dust or blemishes. I'd avoid a lens with fungus (spidery lines) or big chips or deep scratches or corrosion, but otherwise just see how it performs. Outside of having a technician check it on projection, or shooting a test, that's all you can really do.

The important thing with a zoom is the back-focus setting, or distance from rear element to film plane/sensor. This needs to be exactly set so that the lens stays in focus as you zoom from long to wide. If you find the lens drifting out of focus as you zoom out, this is what most likely needs to be adjusted, and should take a lens tech no more than an hour to do, maybe only 15 minutes for some zooms. (Don't focus at the wide end and expect it to stay sharp as you zoom in, you need to focus at the long end where it's more critical and then zoom out.)

charles pappas · November 28, 2017

Thanks to all responders. I am going to auction these lenses on E-Bay as I have only c-mount 16mm cameras, and I did not want to over or under describe them (although i will allow returns).