Re: [Rollei] THE FINAL WORD on Coating Flaws?

[Richard Knoppow <dickburk >]

At 06:07 PM 06/19/2002 -0400, you wrote:
>As I noted some days ago, I sent my 2.8E Xenotar off to John Van Stelten for
>recoating after noticing front element damage.  As we soon learned, I was
>not alone in my observation.  In fact, Douglas Cooper started the discussion
>with the story of HIS 2.8E Xenotar coating flaw.
>
>Well, I heard from John today. The good news is that my lens will "clean up
>fine". Considering that the damage looked pretty substantial to me, this
>should be encouraging to others concerned that the flaws were more than
>"skin deep".
>
>I asked John if he received many mid-1950's Xenotars with similar problems.
>"Yes," he said, "BUT, no more than other German lenses -- Zeiss and Leitz --
>of the same era." In fact, John surmised that he recoats more Leitz 50mm
>Summicrons than ANY other lens...including our precious Xenotars and
>Planars!!
>
>How 'bout that?
>
>Craig
>Washington, DC
>
  This may not be entirely a reflection of coating problems but may be
influenced by the attitude of the owners of various makes/types of lenses
or their perceived value, as well as simply the quantity of various lenses
around. There are lot of Leica lenses out there. 
  Vacuum coating technique was developed in a lot of places. Zeiss was
experimenting with it as early as 1935 and soft coatings were being
experimented with at very much earlier dates. These soft coatings were
applied by immersion of the lens in a solution which left the coating on
the lens surface. Some of these coatings simply rubbed off in ordinary
cleaning. Kodak used a type of soft coating on the internal surfaces of a
few of its lenses beginning about 1940.
  Vacuum coating, or hard coating, began to become generally available
after WW-2. Major manufacturers began to offer coated lenses about 1946
although some of the smaller ones, Goerz, for example, did not generaly
coat lenses for some years after. There were a number of places offering
after-market coating, probably of quite variable quality.  
  I've had a little experience with vacuum coating in the dim, distant
past. Its tricky. I have no doubt that early coating had many problems.
  Modern technique is to give the surfaces a final cleaning by electron
bombardment. Coating is done by evaporating a metallic substance in a
vacuum chamber. For multiple coating several differnet materials are
evaporated successively to very precisely determined thicknesses. 
 The coatings must be uniform and must stick to the glass. 
  The thickness of a single coating can be controlled by visual observation
but there are more accurate methods, and they must be applied to a multiple
coated lens if the coating is to work right. 
  Lens coating fits into the catagory of thin-film technology, a massive
field brought to an exceptionally high degree of advancement by the
semi-conductor industry. 
- ----
Richard Knoppow
Los Angeles, CA, USA
dickburk  

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