[rollei_list] Re: OLD Rollei on Flickr
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Thu, 22 Jan 2009 01:48:37 -0800
----- 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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