Originally posted by Adam I also would try using all 27 AF points. Since the 150-450mm is in the F6.7 to F8 range with the TC, you're pushing the limits of the AF system and might not get the best results with a single AF point unless you're in bright daylight.
I believe your conclusion is the answer. The K-3 may be sensitive to EV(100) -3*, but the prisms used in the PDAF system will begin to "black out"** at about f/8 with the effect being worsened with other than bright conditions. The AF sensels are, quite literally, in the dark. In addition, the lower limit of -3 EV(100) is fairly easily attained in mildly dim light if f/8 is your maximum aperture. Experience has taught me that even above -3 EV(100) the AF performance begins to suffer if the target is of low contrast.
Steve
* It has never been clear to me what EV means in this context. The assumption I have made is that EV is used as a poor substitute for lux (illuminance). By the most generous assumptions, the K-3 lower limit would be taken to mean a meter reading of 30s at f/2.0 (ISO 100) or its equivalent at the same ISO.
** The black out phenomenon is equivalent to the black out seen with split-image and microprism focus aids on an SLR focus screen. Most will begin to fail somewhere between f/5.6 and f/8 with most being unusable by f/11.