Pentax UK and Pentax Germany Admitted K-r Tungsten Front Focusing Issue (But there is No Solution, as Both Clearly Told)
See these two reports and threads. They are 100% self-explanatory, with direct quotes from those Pentax officials:-
Btw, for what I have tested scientifically, that "specific spectrum of light" is called Infra Red. Red light does NOT cause any FF problem to the AF of the K-r, neither does the Green light, which *implies* Yellow light should NOT cause any problem neither! But since tungsten light sources do come with also strong IR spectra. So, it should be the real cause. I have also checked that adding a stronger Skylight 1B filter, which moderately suppresses both Green and Blue lights as well as IR, could lessen the FF very slightly. Thus, the dominant IR in environment should be the root cause to the problem as it is verified and re-confirmed.
After all, it seems that the AF sensors of the K-r are just too sensitive to IR but yet the IR cutting filter used in the AF module is either non-existent or is not strong enough and thus causing this problem - which could never be cured via firmware as it is a *hardware* issue, as I predicted on day one! (but then fanboys rebutted with NO ground and jumped on me, as usual!)
In fact, I confirmed this tungsten AF FF issue at the beginning of this year shortly after I purchased my K-r. I, as many other K-r users who have been suffering from the problem, of course was/were much disappointed for this step back of the SAFOX IX, especially for those who "upgraded" from a previous Pentax DSLR like the K-x and K-7, which do not have this severe issue.
Frankly, Pentax should have created a SAFOX IX-(minus) system instead of what is called the SAFOX+, adding what so-called an additional yellow-light detector won't help them and we end-users, BUT SIMPLY adding an IR cutting filter would! >:-(
Last but not least and still, here is my (repeated) advice for an interim measure to the problem: 1. Turn off the "AF Assist" light in the custom function and then 2. Set "AF Fine Adjust" to -10 (maximum value, my K-r usually focuses somehow acceptably at 0 under daylight for most of my Pentax lenses) and DO *remember* 3. Disable "AF Fine Adjust" after you return to non-tungsten environment and BEFORE you shoot! Easy Enough? :-o
Still Annoying K-5 and K-r Old Issues, Users are Pissed Off!