Many lenses produced from the 1940s through the 1970s are measurably radioactive. The 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. Thorium oxide has a crystalline structural similar to calcium fluoride (fluorite). Like fluorite, its optical properties of high refractivity and low dispersion allows lens designers to minimize chromatic aberration and utilize lenses of lower curvature, which are less expensive to produce. Contrary to often seen statements to the otherwise, lenses containing lanthanum are not appreciably radioactive - lanthanum is only 1/10,000th as radioactive as thorium. Radioactivity in lanthanum containing lenses is due to the intentional inclusion of thorium in the optical glass mix. The presence of thorium can sometimes, depending on the mixture of other elements in the lens, cause moderate to severe browning of the lens element(s).
Typical radiation levels can approach 1 mR/hr as measured at the lens element's surface, decreasing substantially with distance; at a distance of 3 ft. (0.9 m) the radiation level is difficult to detect over typical background levels. For reference, a typical chest X-ray consists of about about 10 mR, a round-trip cross country airline flight exposes a passenger to 5 mR, and a full set of dental X-rays exposes the patient to 10 mR to 40mR.
By far the most prolific producer of radioactive lenses was Eastman Kodak. From the 1940s through the 1960s, substantial numbers of amateur cameras were produced and sold with "thoriated" lenses (containing thorium oxide), including some of the Pony, Signet, and high end Instamatic cameras. In addition, many professional level Ektar lenses from this era contain thorium. Perhaps the most famous radioactive lenses of all were the Kodak Aero-Ektars.
Curiously, in his book, A History of the Photographic Lens, Rudolf Kingslake (head of the Eastman Kodak lens design department 1937-1968), makes only a single passing comment on the possible use of thorium in Kodak lenses.
Kodak lenses tested radioactive (by John Hufnagel)
- Kodak Ektar 101mm f/4.5 (Miniature Crown Graphic camera) lens mfg. 1946
- Kodak Ektar 38mm f/2.8 (Kodak Instamatic 814 camera) lens mfg 1968-1970
- Kodak Ektanar 50mm f/2.8 (Kodak Signet 80 camera) lens mfg. 1958-1962 (3 examples)
- Kodak Ektanar 90mm f/4 (Kodak Signet 80 camera) lens mfg. 1958-1962
- Kodak Ektanar, 44mm f/2.8 (Kodak Signet 30, Kodak Signet 50, Kodak Automatic 35/Motormatic 35 cameras) lenses mfg. 1959-1969
- Kodak Ektanon 50mm f/3.9 (Kodak Bantam RF camera) lens mfg. 1954-1957
- Kodak Ektanon 46mm f/3.5 (Kodak Signet 40 camera) lens mfg. 1956-1959
- Kodak Anastar 44mm f/3.5 (Kodak Pony IV camera)
- Kodak Color Printing Ektar 96mm f/4.5 lens mfg. 1963
Kodak lenses reported elsewhere as radioactive
- Kodak Aero-Ektars (various models)
- Kodak Ektanon 50mm f/3.9 (Kodak Bantam RF camera)
Non-Kodak lenses reported as radioactive
- Canon FL 58mm f/1.2
- Canon FD 35mm f/2.0 (versions from the early 1970's)
- GAF Anscomatic 38mm f/2.8 (GAF Anscomatic 726 camera)
- Nikkor 35mm f/1.4 (early variant with thorium glass elements)
- SMC Takumar 50mm f/1.4 (Asahi Optical Co.)
- Super Takumar 6x7 105mm f2.4 (Asahi Optical Co.)
- Yashinon-DS 50mm f1.7 (Yashica)
Lenses with elements made of contaminated glass
Some lenses of the 1960s have elements made of glass sorts which include small traces of radioactive rare-earth elements. Sometimes this accidental radioactivity causes a significant yellowing of these lens elements. Some users of such lenses reported in camera blogs that they healed the yellowing by exposing these lenses to the ultraviolet light of the sun. The procedure needs several days of sunny weather to have a positive effect. Lens elements with such yellowing radioactive impurity are in the following lenses:
- Minolta MC W. Rokkor-SI 1:2.5 28mm (early variant, before radioactive glass impurity could be banned)
- Minolta MC Rokkor-PG 1:1.2 58mm (early variant, before radioactive glass impurity could be banned)
The healing of yellowing by sunlight is also reported for some lenses with thorium glass elements, for example for the Nikkor 35mm f/1.4 lens and the Super Takumar 50mm f/1.4 lens.
- The Aero Ektars
- Thoriated Camera Lenses
- Health Physics Society
- Office of Civilian Radioactive Waste Management - Fact Sheet
- Rudolf Kingslake, A History of the Photographic Lens, Academic Press, 1989, Chapter 5, section 4