There is a curious phenomenon which affects some older lenses as
they age: the cement used to bond glass elements together can turn yellow.
While comparing two of my recently acquired M42 lenses with each other,
a 50/1.4 Super-Takumar to a 58/2 Helios-44-2, I was puzzled when meter
readings from the Super-Tak called for a slower shutter speed on the same
subject with both lenses wide open.
I also noticed the view through my Spotmatic's finder was yellow-ish
with the Super-Tak. Some research on this topic turned up that this is a
common issue afflicting these lenses.
Others report success bleaching the lens back to clarity using ultraviolet
light, either from direct daylight (window glass blocks UV) or via a strong UV light
bulb. I decided to try a CFL party black
light and heat lamp fixture from home depot.
After merely 24 hours of exposure to UV there was a noticeable
difference, and by 4 days later it appeared cleared up to the naked eye.
My meter readings now called for a one-stop
faster shutter on the Super-Tak so I figure I am done with the UV light treatment.
Before and After: Helios 44-2 on the left and Super-Takumar 50/1.4 on the right
As an added bonus I now have a black light at my disposal just in time for Halloween trick-or-treaters!