Originally posted by troenaas The unmarked button. What can it be? Let me trove a speculative suggestion into the discussion.
It is to activate a blocking filter / closing of the viewfinder in order to avoid light-leaks during long exposures.
Nikon have a curtain in the viewfinder for this.They have a small lever to activate it near the eye piece.
We all agree that the screen is touchscreen? But what if it was simply turned on by pressing the menu button and off by touching an icon on the screen?
This would mean the proximity sensor was for another purpose. i.e turning a power sapping electronic display on and off in the viewfinder as required?
The button could be to select CDAF / PDAF with in viewfinder focus peaking? After all on the K-1 it is the Live View button.