Archived posting to the Leica Users Group, 1997/11/04

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Subject: Re:Adi /Yellow Cast/Thorium glass
Date: Tue, 4 Nov 1997 21:13:48 -0500 (EST)

 Adi - It is still a controversial subject, since at first Leitz denied ever
 using radio-active Thorium in their glass to make the early 50/2 
 Summicrons with s/n under 1,000,000.  It is now common knowledge
 that they and Hassleblad & others did.   Dr. Cyril Blood in the UK was
 one of the first to write about this in the LHS Newsletter.  With age, this
 glass turns yellowish brown & will show on Chromes.  Bill Gordon & I
 followed up on Dr. Bloods request for info and found that I had 2 of them
 in my collection and Bill had 3.  Checking them with a sensitive Geiger
 counter, Bill found that neither his nor mine exceeded 2.5 mili-roentgens
 on the front element & 1.5 m.r. on the rear;  no where as near as the old
 radium watch dials of the 1950's and would take about a week in the dark
 on top of a piece of film, to leave an exposure. All this, and photos also,
 were written up less than a year ago in the LHSA's Viewfinder.
   There was one other type of so called "yellow glass" that Alf mentioned
 in regards to soft & hard coating of the Summicrons; that was a very soft
 magnesium flouride coating that was used for awhile but was discontinued 
 by Leitz in favor of the harder magenta coating. All the Leica lenses that I
 have had polished & recoated by John van Stelten have had this hard coating.

 Marvin Moss
In a message dated 97-11-03 12:53:56 EST, you write:
 I looked through my 50 f/2 lens at a white piece of paper and found that
 there was a strong yellow cast to the image. I tried this also with an 80mm
 Planar for Hasselblad and it too had this yellow cast. Is such an
 occurrence normal ?