I believe the second little button on the back of the lenses first
appeared on Super-Multi-Coated Takumar lenses when they were still M42
thread mount. This configuration changed nothing when these lenses
were introduced with the Spotmatic II bodies. The new bit of machinery
inside the lenses was done in anticipation of a yet to be Pentax that
would be called the Spotmatic F.
When screwed onto an F type Spotmatic, SMC lenses allowed open
aperture meter reading for the first time on a Pentax 35mm camera.
Such lenses could still be used on Spotmatics and Spotmatic II bodies,
but these lenses need be stopped down to run the built in meter by
turning on the meter switch.
I still use Super Takumar, Super-multi-Coated Takumars and more modern
K mount Takumar lenses on my digital Pentax camera. I run the camera
in manual mode and switch the focus to the manual setting. Usually it
only takes a shot or two to get an acceptable exposure set-up. I will
also set the focal length of the lens, times 1.5, and leave the motion
reduction system in the ON position.
Richard Bush
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