(Source: Pentax Japan K-5II/s page, Credit: Blog reader "jaad75".)
Look at the above official diagram and illustration. For the "new" K-5II and K-5IIs, the first thing is that only the central AF point is equipped with the f/2.8 sensor, for the horizontal one. That f/2.8 sensor is unique for being the one and only one f/2.8 AF sensor that the camera has. However, this sensor will NOT work with any lens that is slower than f/2.8! Under such case, the horizontal AF sensor will have no function! That means that the central cross sensor will simply become only a vertical linear sensor with most zoom lenses, except for those very few f/2.8 ones. Moreover, even those slower primes are affected, e.g., the DA21 F3.2 Limited and the DA15 F4 Limited!
At the end of the day, I would rather keep my K-5 and do the AF fine adjustment in a precise way instead if I have to sacrifice like that, which is even more problematic IMHO! :-o
After all, my last prediction is correct! Pentax is yet to do thing in the easiest but cheapest way, without taking the real drawback caused as a result into account! >:-|
Related:-
K-5II User Manual is Put Online
What is the Exact Layout of the f/2.8 AF Sensor(s) in the K-5II/s?
Focus Adjust the K-5 and Pentax AF Lenses with Live View
Again, Why a "f/2.8" AF Sensor is Crucial for Higher AF Accuracy!