[Leica] DR contrast and the 'rare earth' Summicron

[sethrosner at nycap.rr.com (Seth Rosner)]

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