Re: [Leica] 85mm f2 Russian now a question of Thorium glass

["animal" <s.jessurun95@chello.nl>]

There is good info on Thorium glass here:
http://home.earthlink.net/~michaelbriggs/aeroektar/aeroektar.html
simon jessurun

- ----- Original Message ----- 
From: <P2CON@aol.com>
To: <leica-users@mejac.palo-alto.ca.us>
Sent: Friday, January 16, 2004 8:23 PM
Subject: Re: [Leica] 85mm f2 Russian now a question of Thorium glass


> Don, and other interested parties,
>
> I have an example of each; the LTM version, made by KMZ in 59, and the
> Kiev/Contax mount, made by LZOS in 61. Both show a very yellow reflection
from one
> of the interior elements, along with a pale yellow/white behind and in
front of
> it and the very bright purple reflection from the front element. These two
> lenses from two different factories show the exact same reflections when
held
> against the light. When looking through them at a white sheet of paper
there is
> a slight yellowish cast to the image. I have never noticed any effect on
an
> actual photograph taken with either lens, apparently the yellowing is not
> sufficient with my examples to be noticed, or else I am not that color
critical.
>
> My experience with glass yellowing badly is with the Pentax Super
Takumars.
> The Pentax enthusiasts have discovered that the yellowing can be reversed
by
> soaking the lens in direct sunlight for a couple of weeks. I did try this
with
> one really bad 35mm/f2 that was unusable with color film due to the
yellowing.
> Two summers ago I put it out on the patio with the sun shinning directly
> though it. I wrapped the lens barrel with aluminum foil so that the sun
light was
> reflected back through it and pretty much kept it oriented so the sun was
> shinning in on the lens axis. I quit the experiment after about ten days
with the
> yellowing pretty much gone. Just now I took it outside with one of the
Soviet
> 85/2's and checked both against a sheet of white paper. The Super Takumar
still
> shows less yellowing that the Soviet lens. As I said before, there is no
> obvious color effect on film with the 85's, but maybe next summer I will
try the
> sun soaking trick on them anyway.
>
> Perhaps some of you with yellowed Summicrons have tried this trick. Anyone
> willing to tell us about it?
>
> Regards, Paul Connet
>
>
> << Don writes:
> << Karen,
> You make an excellent point and I have some older lenses where the
> balsam is clearly yellowing.  However, this particular lens has at least
> one very yellow element, probably the thick center element in the
> triplet of a sonnar design.  In this case, far more yellow than the one
> in my very early Summicron or in any of the aero Ektars I have seen.
>
> I have only seen one other lens almost this yellow and that was one of
> the early production Canon 35 F2 FD lenses.
>
> Don >>
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