Other common misconceptions hold that it is the coatings of these lenses which are radioactive and brown over time, and that the browning occurs in the balsam between cemented elements. The presence of thorium can sometimes, depending on the mixture of other elements in the lens, cause moderate to severe browning of the lens elements. Radioactivity in lanthanum containing lenses is due to the intentional inclusion of thorium in the optical glass mix. Despite statements to the contrary, lenses containing lanthanum are not appreciably radioactive - lanthanum is only 1/10,000th as radioactive as thorium. Like fluorite, its optical properties of high refractivity and low dispersion allows lens designers to minimize chromatic aberration and use lenses of lower curvature, which are less expensive to produce. Thorium oxide has a crystalline structural similar to calcium fluoride ( fluorite).

Main source of radioactivity is the use of thorium oxide (up to 30% by weight) as a component of the glass used in the lens elements. There are a significant number of lenses produced from the 1940s through the 1970s that are measurably radioactive. 5.2 Lenses Tested non-Radioactive (by contributors to this page).5.1 Lenses Tested Radioactive (by contributors to this page).4 Lenses with elements made of radioisotope-containing glass.

Pdf vivitar series 1 70 210mm f2.8 4