I am posting this in hope that someone knowledgeable will enlighten me (and others, I am sure).
Background: The new K-5II has a new autofocusing system that, according to Pentax, includes F/2.8 sensors in addition to the F/5.6 sensors. The sensitivity is said to be -3 -- 18 EV. The old autofocusing sensor was specified at -1 -- 18 EV, I think. In other words, the low-light sensitivity has been increased by 2 EV.
My assumptions: The original K-5 had F/5.6 auto-focus points and it was specified from -1 EV.
My observation: The difference between F/2.8 and F/5.6 is 2 EV -- the same as the improvement in low-light auto-focus sensitivity.
My conclusion: The improvement in auto-focus low-light sensitivity stems fully from the step to F/2.8 focus points.
Questions:
1) Is my assumption that the original K-5's auto-focus points are F/5.6 correct?
2) Is it as simple as I assume: otherwise identical auto-focus points would improve their low-light performance by 2 EV when increasing the size from F/5.6 to F/2.8?
3a) Does this imply that when used with lenses slower than F/2.8, the camera would use the F/5.6 auto-focus sensors only, and then have the same -1 EV low-light sensitivity as the original K-5?
3b) Or, could the F/2.8 sensor aid the F/5.6 sensor for improved low-light performance if a lens-size in-between was used, like for example F/3.5?
Naturally, there are likely also other improvements of the new auto-focusing system. As an example, since the K-30 had improvements in the form of some kind of light diffuser to decrease the risk of the auto-focus color temperature sensor giving false readings, I would assume that such a diffuser is also on place on the new system. New algorithms were also mentioned in the press release (
PENTAX - PENTAX Introduces New K-5 II & K-5 IIs DSLR Cameras).
I would be grateful if someone could explain this in more depth to me.
/Jonas