Canon Lens FD 35mm F/2.0

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Canon FD 35mm F/2.0 Family Portrait



Six different version of FD 35mm F/2 lenses are known. The following table lists the key features of the lenses.

Lens Version Optical design Front Lens Convexity Min aperture Mass
(g)
Thoriated (radioactive) S/N range Coating
(*)
First datecode
FD 35/2 I (chrome nose) 1 Concave 16 420 Yes 10001-12199 - March 1971
FD 35/2 II (chrome nose) 1 Concave 16 420 Yes 12200-20000 - Approx. May 1971
FD 35/2 III (chrome nose or black nose w/o SSC) 1 Concave 16 420 Yes >20001 - Approx. August 1971
FD 35/2 SSC I 1 Concave 16 370 Yes >50000 S.S.C March 1973
FD 35/2 SSC II 2 Convex 22 345 No >100000 S.S.C April 1976
FDn 35/2 New FD 3 Convex 22 245 No S.S.C December 1979

(*) According to certain references all the chrome nose 35mm F/2 lenses were SSC coated


In the image above there is a study of the serial numbers and timing based on datecodes.

The radioactive lens

Radioactivity in the first four 35mm F/2 lenses on the table is caused by thoriated element(s?) on the lens.
Thorium is a metallic element belonging to rare earth elements. Thorium was mixed into glass mass for improving the index of refraction while remaining low dispersion (typically high index of refraction leads to high dispersion). It allowed manufacturers to make thinner lenses. During the 1970's the usage of thorium was dropped and replaced by other optical desing.  

There is discussion in certain web resources if the radioactivity element is thorium or lanthium, which is another rare earth element that also was used because of the same reasons.


Level of radioactivity


Measured by using Master-1 dose meter (Geiger-Müller counter) the dose rate is 1.31 μSV/h (at the front-lens end) and  up to 3 μSV/h (on the breechlock end). My assumption based on this measurement is that it is the rear element is the thoriated one. The dose rate is more than 20 times the amount of normal background radiation in Kgs. Lyngby, DK. The radiation is faintly measurable at the distance of 0.5 m from the lens. There is no significant difference in the radioactivity between the four different designs based on thoriated element.

Carrying this lens close to you 24 hours/day would give a dose of approx. 26 mSV per year. As a reference the Finnish nuclear power plant employees maximum allowed dose is 50 mSV per year and the yearly awerage must not be more than 20 mSV over five years period.


The radioactive lens in use


The thoriated elements turn brown over time. All the 70ies lenses I have seen have been quit yellow/brown. This saves the B/W photographers need to invest in yellow filter but color slide film scans must be photoshopped in order to remove the yellow cast. The process is based on physical phenomenom and causes the whole element to change color - not the coating or surface only. I am still working to investigate the spectral effect of the lens to understand the best way to correct the yellowness. Light threatment removes the yellowness at least partly: placing the lens in direct sunligth over some weeks or months makes is clear again.


Vesa Vähälummukka 2008-2015