Film-Tech Forum ARCHIVE: Lens Degradation

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Posted by Paul Stevens (Member # 4016) on 05-04-2007, 02:55 AM:

I recently swapped out a flat lens because I noticed that micro pitting was forming on the inner surface of one of the rear elements. In addition, there was a slight opaqueness as well. After replacing the lens there was an immediate and dramatic improvement in projected picture quality. Not a surprise but I was delighted to see it none the less. Another candidate in our house is the anamorphic lens mounted on the turret of a Kinoton PK-60 used for the Ultra Screen. Nearly the same thing noticed on the rear elements.

Over time most projector lenses must degrade to a point where something must be done. If your lucky, there is an abundance of suitable replacement lenses to swap out allowing the degraded hunk of glass to be sent off somewhere to be rejuvenated.

Has anyone had similar experiences?

Posted by Chris Slycord (Member # 4239) on 05-04-2007, 05:08 AM:

Stupid Question:
Were (are) your lenses getting cleaned regularly?

Posted by Kenneth Wuepper (Member # 1174) on 05-04-2007, 07:03 AM:

Paul,
It sounds like you have "cemented" lenses. That is there are two elements of glass cemented together to make one optical unit. This cement eventually degrades and discolors. Sometimes excessive heat may cause the cement to decompose and make a haze inside the lens.

Recent computer designed lenses make possible the "air spaced" design. These lenses eliminate the cement and the problems that come from that technique of construction. The air spaced lenses can withstand much more heat and the image is visibly better than older cemented type lenses.

Posted by Paul Stevens (Member # 4016) on 05-04-2007, 12:14 PM:

Not a stupid question at all Chris. Sensitive may be but not stupid by any means. No offense taken. Yes the lenses are cleaned weekly and when ever air gets blown around. We are pretty careful about that.

Kenneth. Not sure about the use of cement unless it is used during fabrication to keep the locking rings from working loose. On that flat lens I replaced, the rear ring kept working loose. This may have allowed lens cleaner, or any number of substances to work there way inside to vaporize by heat and coat the inner surfaces and contributing to opaqueness.

The little dots or pits I referred to are puzzling. When light hits them, they of course glow and I am sure do some refraction that does not help color very much. Interestingly, after removing the first rear element on the lens I replaced, (it was loose anyway), the damage nearly at the center of the second element looked like a surface abrasion. I did not have a microscope to examine it in detail. What ever coating there was looked like it was decomposing some how.

What is another curious item is the difference in age between the lenses mounted on the Strong, and those of the Kinoton. The Strongs are nearly ten years old while the while barley two years on the Kinoton. Also, lenses on the Strongs have been running first 2KW and now 3KW lamps. The Kinoton is running 7KW.

The huge problem with doing anything with the lenses on the Kinoton are they are one of a kind to the extreme, so finding a substitute or replacement will be very costly indeed. Not the kind of news the bean counters want to hear.