Originally posted by desertscape It was only the Super-Takumar 105 that used Thorium glass and they are not that common now. It was the only lens that Pentax used Thorium glass.
The M42 Takumar 50mm f/1.4 also has a Thoriated lens element in it*. The Super-Takumar 105mm f/2.4 wasn't the only lens from Pentax that has developed a undeserved reputation for being
dangerously radioactive**. If you kept one of these lenses in your pocket and wore those trousers 24/7 for a few years: then yes, your chances of developing cancer will be
elevated but it wouldn't kill you....at least not directly***.
Originally posted by First Poster When did Pentax switch over to non-thorium lens elements?
Circa 1968~ 1973 - when chemically pure Lanthanum salts were able to be manufactured on an industrial scale.In later years Lanthanum was eventually used as a substitute for Thorium. Eventually the process for refining the ore, Bastnäsite, which does contain considerable amounts of lanthanum (without thorium) helped improve processing to the point that it was no longer tainted by radioactive Thorium.
Radioactive element from an SMC takumar 50mm f/1.4 made circa 1966
* There are several actually, though there is a degree of debate over whether the addition of thorium in the lenses was intentional, as Lanthanum is easily tainted by Thorium as both metals are commonly found in the same ore, Monazite (which contains along with other rare earth metals, both Thorium and Lanthanum). With 1950s technology chemically pure Lanthanum was difficult to make on an industrial scale, such cross contamination was pretty much inevitable.
**Unlike some of the old Kodak Aero lenses, which are fully deserving of it.The pre-1950 Aero Ektar lenses are vicious alpha particle emitters due to heavy amounts of thorium used in the glass. Two elements found in the the two 7" and 12" f2.5 Aero-Ektars are made of glass with
11% and
13% Thorium by weight (bloody hell Kodak). While I don't have the study paper in front of me I recall that at most, 1960s era lenses from other 35mm lens manufacturers: Leica,Zeiss,Olympus,Pentax et.al only had 3~5% Thorium at most.
*** needless to say, I am not an expert on radiation. Take my advice with a brick of lead.
Last edited by Digitalis; 09-24-2015 at 06:39 AM.