Ricoh PENTAX K-3 Mark III Faq page 49

Restrictions on the Selection of AF Point
When the following lenses are used, the AF points on both sides cannot be used while
shooting with the viewfinder.
Q: Is it possible to take a picture with set the aperture ring to a position other than
the A position?
A: Yes, the shutter can be released even if the aperture ring of the DFA, FA, F or A lens
Is not set to the A position or a lens without as A position is attached.
Set [Using Aperture Ring] to [On] in the C6 menu.
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