The following article is reproduced from the Spotmatic Group with the kind permission of the Author Ray Allen

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How to Cure Yellowing in Takumar 50mm f1.4 lenses.

Remove the lens cap and wrap the uncapped lens, except the rear element, in aluminum foil and  then set it on a window sill that faces towards the sun. Prop it up with something (like a rolled-up towel) so that it tilts in order to get the most sunlight into it.  Then go away and leave it. A mild case takes about a week to ten days on a window sill in California. A severe one takes about a month.

The foil serves two functions.  It reflects light off the lens body and thus reduces heat buildup that otherwise would occur due to the black finish on the lens and this could affect the lubrication Inside the lens. And by not capping the lens before wrapping it, light makes its way from the back of the lens and reflects back off the foil at the front of the lens, thus attacking the yellowing again. 

This "sunlight cure" method has been successfully employed by a number of members of the Spotmatic group and many 50mm f1.4 lenses which were thought to be useless for colour Photography have now been returned to active service with "water clear" glass. 

Some others have reported good results in about three weeks using an ultraviolet light source.

The yellowing problem affects the Model II 50mm f1.4 Super-Takumars and all other 50mm f1.4 lens through to the K-mount series. The older Model I 50/1.4 Super-Takumars, the 8-element ones (which can be distinguished by the protruding element at the rear NOT having a protective metal rim), do not turn yellow because they do not have the radioactive element, using an Additional regular optical glass element to get higher refraction instead.

Apparently the atomic structure of the glass is altered by radiation from trace amounts of radioactive thorium in one rare-earth glass element, creating the yellowish-brown color; and this change is reversed by exposure to ultraviolet light.  A good link to a scientific explanation is available here:

http://medfmt.8k.com/mf/radioactive.html

And see an interesting experiment with a Super-Multi-Coated Takumar here:

http://home.att.net/~jriegle/radioactive.htm

Bryan Ayling has posted an alternative repair procedure for this problem at:

http://www.hermes.net.au/bayling/repair.html

Use this procedure at your own risk.

Ray Allen.

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Copyright Kim Coxon