Certain lenses produced from the 1940s onwards used a type of optical glass that
included traces of radioactive rare-earth elements. This was apparently done
to alter the refractive index of the glass and enable design of fast lenses
with improved optics. As some lenses aged this glass turned yellowish in colour,
with a detrimental effect on performance.
Of the screw-mount Takumars produced by Asahi Optical Company for 35mm cameras, yellowing has been commonly observed in the f1.4/50mm and f2/35mm fast lenses (both Super-Takumar and SMC Takumar). In the worst examples, a strong colour cast is noticeable when looking through the lens, and affects the colour of photographs.
The problem should not be confused with the distinctive amber colour of coatings found on many Super-Takumar lenses, which does not seem to impart any colour cast. Also be aware that the majority of Takumar lenses do not use the affected glass.
To check if the lens has yellowed, remove any filter and with the iris wide open, hold the lens up against a bright background (white or light grey). Colour of background seen through the lens should differ little from that viewed directly. In the example shown here the yellow colour shift of this defective lens has been exaggerated slightly.
The repair outlined here was made to a Super-Multi-Coated TAKUMAR 1:1.4/50mm
screw mount Pentax standard lens.
Prepare a solid, flat workbench and select your tools. For this job, the largest implement was deemed most suitable.
Repair the lens by following the procedure shown in the sequence of photos below.
Repeat above step, if necessary.
The repair, if successful, ensures that the defective lens is no longer compatible with your screw-mount Pentax. In any case, yellowed lens elements should no longer be present.
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All photos on this page are by Brian Ayling and Myles Harris-Ayling, using a Pentax Spotmatic II and SMC Takumar 1:2/55mm, exposed on Kodak Gold 100 colour print film.