[rollei_list] Re: OLD Rollei on Flickr
----- Original Message ----- From: "Elias Roustom" <eroustom@xxxxxxxxxxx> 
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Wednesday, January 21, 2009 5:24 PM
Subject: [rollei_list] Re: OLD Rollei on Flickr



Have any of you seen these pics:
http://www.flickr.com/photos/photoyann10/page2/
?

Really great.

Enjoy,

Elias
Someone on the blog commented about lenses becoming "self coated" over time. This is correct and is probably what the site owner is observing. After many years the surface of some types of optical glass oxidize enough to produce a coating of material with a lower index of refraction than the glass. This acts as a sort of coating. If its dense it can look blue, if not very dense it can have a rainbow appearance similar to an oil slick. The front elements of early Tessar lenses are vulnerable to this kind of oxidation. However, it depends on what kind of atmosphere the lens was exposed to. For instance, I have two 135mm, f/4.5 Zeiss Tessars on Speed Graphics, both dating from about 1939, one is completely clear the other has a strong blue cast on the front surface. Generally oxidation affects mostly the front surface, the inner surfaces and the surface inside the camera are protected enough to prevent the effect. H.D.Taylor, the inventor of the Triplet, discovered that "tarnished" lenses passed more light than freshly polished ones. He was able to determine the cause but was not very successful in reproducing the effect with chemical treatments. This was sometime about 1890, it was not until the mid 1930's that anything like a satisfactory coating was devised and not until the mid 1940's that "hard" vacuum coating was developed which made coating possible on a production basis. 

--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 
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