Focusing fast lenses wide open.

Some heretical thoughts :

The stock Pentax viewfinder screen is really cr*p with lenses faster than about f2.8. A cut down Canon Ee-s screen makes all the difference.

I've always used centre focus and recompose - old habits die hard. I could never see the point of having dozens of AF focus points and the camera deciding for me what I should focus on.

On Pentax cameras later than the K-5 centre focus and recompose movement tends to reset the SR and you have to wait for it to lock in again.

For testing it's worth trying out the Live View. That shows exactly what's hitting the sensor - so is helpful in checking what the lens can actually do

Once you have that referenced you can work from there... and possibly add in some focus adjustment to correct the phase detection autofocus and bring it in line with the contrast detection.

Be aware though that K5 and prior are quite sensitive to lighting colour temperature changes when performing AF. The K5ii/iis made a huge difference for AF accuracy on fast glass. But, as I found out the other day there are still situations where things can go wrong (backlit dancer in poor fluorescent light was a right pain!)

Matt