Originally posted by Wired I'm 100% certain now I got a bad copy of the Tak. I got a Fotdiox adapter for my D600 today and mounted the lens up. I did shots with the F 50 1.7 and the Tak 50 1.4 on both the D600 and the K5 using similar exposures. the F is crystal clear. The Tak has liver faliure.
You probably have the "radio-active" version of the Takumar 50/1.4. Thorium was used in the making of the lens glass elements, and over time, the lens glass goes yellow (you can google this, or search on PentaxForums). The solution is to place the lens (with lens caps removed) in strong UV light for a few hours, or strong sunlight for a few days, and the yellowness will fade (for a few years or so).
The radioactive levels are generally safe, just don't walk around with it in your pants pocket for months and expect to have kids down the track!
In the B&W era, the yellowness of the lens was seen as an advantage, like taking shots with a built in yellow filter to enhance contrast.
On a separate note, the fotodiox adapter has a 1.4x adapter lens built-in, you may lose image quality because of this (as well as gaining magnification, and losing 1 stop of aperture).
John.