Originally posted by biz-engineer You mean that face detection is handled by the AE sensor rather than PDAF sensor matrix?
Yes. It's been that way for a while with Nikon, even with AE sensors of smaller capability than the 91k metering sensor on the D4S/D800/D810/750. Pic related, for the D800, showing face detection being done by the AE sensor (for OVF), as well as by the image sensor (for EVF).
With Pentax, since the K-3 arrived with it's 86k metering sensor, the camera EXIF has been showing face detection results using the OVF, but face detection has never been turned 'on' by the firmware for use in OVF focussing, unfortunately. It works fine in LiveView, of course, but not OVF. IMHO, if the hardware is able to support OVF face detection in Pentax, they should enable it.
Many people on these forums have criticised optical viewfinder face detection assisted AF as un-necessary and a gimmick, and hence not worthy of inclusion in any serious Pentax like the K-1, but it is genuinely useful in real-world [and pro] use. I wish Pentax would make it work in the K-1.