Here is a list of the Takumars which can turn yellow:
Which Screw Mount Takumar Lenses Can Turn Yellow?
The following screw mount Takumars are known to yellow:
Part No. Model
--------- -------
43931 1:2/35mm Super-Takumar (49mm filter)
43932 1:2/35mm Super-Multi-Coated Takumar (49mm filter)
37800 1:1.4/50mm Super-Takumar
37801 1:1.4/50mm Super-Takumar
37802 1:1.4/50mm Super-Takumar
37902 1:1.4/50mm Super-Multi-Coated Takumar
37982 1:1.4/50mm SMC Takumar
The earliest 1:2/35mm Super-Takumar (part nos 368, 43680, with 67mm
filter) and 1:1.4/50mm Super-Takumar Model 1 (part nos 358, 35800)
with 8 elements) are NOT subject to yellowing.
The 1:1.4/50mm Super-Takumar Model 1 8-element lens can be easily identified by:
• Looking at the position of the red infra-red focusing mark which
lies between the f4 depth of field marker and the infinity mark.
On the later 7-element lenses the infra-red focusing mark lies
between the f8 and f4 depth of field markers.
• The protruding rear element which lacks the metal retaining ring
found on the later models.
There are many more 7-element lenses than the 8-element version.
No other screw mount Takumars are known to suffer from yellowing.
And here is how to cure the "problem":
How to Cure Yellowing in Takumar 50mm f1.4 lenses.
Remove both lens caps and wrap the uncapped lens, except the rear element,
in aluminum foil and then set it on a windowsill 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 windowsill 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. The Yellowing problem also affects the later
f2 35mm lenses with 49mm filters.
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.
Bryan Ayling has posted an alternative repair procedure for this problem at:
Brian Ayling's photographic repair tips
Use this procedure at your own risk. Please note that it is not reversible.