Hi,
What you are seeing is the same with what I saw when playing with my friend's new K-50.
- Green button metering using the optical viewfinder sensors fails in M-mode when using lenses having a non-conductive base such as your SMC Takumar
- Green button metering in M-mode works fine for Liveview* with all non-A contact lenses
- Green button metering works fine for the optical viewfinder with all K-mount lenses
- A-series and newer work in all modes/views when the aperture ring is on the A setting
You might want to consider using Av mode with your M42 lenses. While the manual suggests that the metering may not be reliable in that mode, I did a quick test and exposures seemed to be reasonable and comparable to those done in Liveview.
Steve
* Metering in Liveview is based on the measured response from the camera's image sensor. Viewfinder metering, on the other hand, uses the SPD sensors in the prism housing that meter off the image projected on the focus screen. The two systems differ in how they work with lenses lacking support for body-controlled aperture ("A" contacts).