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bjornthun wrote:
Today lanthanum is not used in the apo lanthars, as new materials are used to achieve apochromatic performance.
Whether or not C/V uses lanthanum in their house melts, it remains a component of commercial optical glass. Schott products with lanthanum as the primary additive have "La" in the name. So, N-LAK10 is a lead-free crown glass with La. Elemental additives like fluorine, phosphorus, lead, arsenic, barium, boron, are used to tune the optical properties of the base glass.
https://us.shop.schott.com/advanced_optics/Products/Optical-Glass/Optical-Glass/c/optical-glass/optical-glass
Radioactive lenses like the infamous Pentax 50/1.4 were nominally constructed with La glass but the mining process includes another element, Thorium, which is a weak alpha emitter. Those particles are unlikely to escape the lens, let alone penetrate your body, but does yellow the glass over time to a significant extent. Like Pb (lead), Th is a real hazard in lens grinding activity. Modern optical La2O3 is free of Th contamination.
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