Archived posting to the Leica Users Group, 2010/09/25
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Phil & others, I believe you are confusing two different things. Some of the early 50 Summicrons ( they were all collapsible) were manufactured with glass one of whose metallic 'rare earth' elements was thorium. It was this rare earth that resulted in the yellowish tint and the slight radioactivity emitted by a number of the elements. Leitz recomputed, more accurately re-designed the 50 Summicron, i.e. the 1956 DR/Rigid Summicrons, with what they described as thorium-free glass. These lenses had glass elements with the 'rare-earth', lanthanum, described in the literature as LaK9 glass. A reason I occasionally used to sign off on an LHSA communication: LaK9 = Love and Kisses 9 times. ;-) Seth