Originally posted by Prakticant Does it mean other AF points are not so much helpful with 2.8 lenses?
Or opposite, this f=2.8 AF points work only with fast glass?
The first is closest to the truth. The concept is called focus sensitivity and is expressed as f-number. The optical physics are a bit complicated, but it is enough to say that the general standard for PDAF is a focus sensitivity of f/5.6.* What that means is that the focus precision (chance of actually attaining accurate focus) is no better at wider apertures than at f/5.6. There are huge implications since the DOF is much deeper at f/5.6 than at say, f/2.8 meaning that with faster glass, missed focus is much more likely at that maximum aperture than with slower lenses. Note that this nothing to do with the amount of light present, the precision limit is the same for both bright and dim conditions. The focus sensitivity is driven by the aperture size alone.
PDAF systems with one or more f/2.8 sensors are pretty much the domain of upper end cameras. Both the K-5 (one at center) and the K-3 (3 at center column) have f/2.8 sensors. The f/2.8 sensors have the same focus precision at f/2.8 as at wider apertures and and still work with slower glass. With lenses faster than f/2.8 there is still a loss of precision relative to a split-image or Canon S-type focus screen and potential for missed focus, but that is just the way it is.
As noted above, CDAF (liveview AF) can be more accurate, but is slower and may fail to attain focus in dim light. My K-3 requires the AF assist lamp for CDAF in light where the PDAF system functions fine without it. As for focus shift on stop-down...yes, it exists, but it is usually offset to a degree by increased DOF. The lenses I own that shift have about 3-5mm change at 1m distance from f/1.4 to f/5.6. In practice, the impact of focus shift for AF lenses is the same as for legacy manual focus lenses. As always YYMV.
Steve
* I have long felt that the general scarcity of fast AF glass in the Pentax line was intentional and due to the limitations of the AF system. Note that this is not just a Pentax problem. Back in the day, most cameras shipped with a f/2 or faster 50mm lens and a viewfinder that was equal to the task. Current cameras ship with f/3.5-5.6 zooms and viewfinders that are no better than the AF system.