http://forums.dpreview.com/forums/readflat.asp?forum=1036&thread=35758153
Sample photos for the focusing errors are posted for times along the thread, which are shown up as solid evidence. And, there are some other posters replied do second his view and the same disappointment is expressed.
In fact, what causes the problem? Why? Below are a number of major causes that I can think of, namely:-
1. Inadequate Design(s) of the SAFOX VIII/+ AF System:-
1.1 Large AF Sensing Areas:
First of all, the sensing area of the AF line sensors are just too large! In fact, it is just so large for the area of coverage that the user by no means could pinpoint the focus more precisely:
As you can see, the red lines indicate the actual location of the AF line sensors. The central AF bracket mark provides only a rather rough indication for where the AF actually measures. But still, the big problem is, the line is just too long and the user can never select and pinpoint the targeted focusing point more precisely. Under some more critical situations, e.g., when using fast prime at closer distances, this is just a real issue that never has remedials. The user is just helpless.
1.2 Decreased AF Sensor Resolution (Possibly):
Secondly, the magnified AF line sensors yet cause another no-so-obvious issue. That is, with the same AF line sensor and module which has a fixed number of pixels used (i.e., the pixel density), the larger the AF area that it is magnified, the lower the effective resolution and thus the accuracy of the AF sensor and measurement will be, for the image that is projected through the split-prisms of the AF module on that specific AF sensor. Of course, one can argue that Pentax might have put a very long AF line sensor with yet effectively high enough pixel density even the AF line coverage is large. But then this is very unlikely as it would increase much the cost of the AF components and thus the overall manufacturing cost.
1.3 No "f/2.8" AF Module:
Without putting a "f/2.8" AF module which detects light (paths) coming from the peripheral of the larger aperture lenses, the spherical aberrations of fast lenses cannot be really measured (but just assumed) and thus the AF accuracy suffers. Sometimes I just wonder, even entry level Canon DSLRs (since the 450D) have had this for long, so as to improve AF reliability for faster lenses with larger apertures, why Pentax has still been unable to put such a really standard design and solution so as to tackle the problem in a more effective way?
1.4 Yellow Light Symptom:
I found that the symptom has not been so obvious with the new Pentax DSLRs like the K-x and K-7 (provided that they are fairly "accurate" enough by themselves for a specific body and lens under a specific condition) but the problem is actually not totally eliminated. To check, just project a white light on an object, fix the camera, do the AF, change the light source to yellow (a 2700K to 3000K one), then re-do the AF, read the focus scale on lens, then you can see the camera should usually focus a bit fronter and then Front Focusing is resulted if you inspect the photos! :-o
2. Inferior Quality Control:-
Without doubt, from my own experiences of possessing 7 Pentax DSLRs over years myself (2 *ist Ds, 2 *ist DSes, a K100D, 2 K-xes - not even count for many other Pentax DSLR bodies and models that I have used and tested), I am totally and 200% "impressed" with the Quality Variations of individual Pentax DSLR bodies (and lenses as well) at the Pentax factories. With different units of bodies and lenses and then combinations, the focusing errors and behaviours could vary in large extents and unpredictable ways. So, the interim solution for we poor Pentaxians is just the Debug Mode, especially for those entry level or older model users. But then this is only a partial and incomplete solution, as different lenses could have different errors again and AF accuracy is also affected by the distance of the objects, even your Pentax DSLR(s) has/have been debug mode adjusted! :-(
Still, below are those Debug Mode tutorials and related posts that I have put in my Blog so far, hope this can be somehow useful for you to "cure" the problem, which I believe that for some users of "advanced" models like K-7 and K20D would still need it for some cases:-
http://ricehigh.blogspot.com/search?q=debug+mode
3. The Negative Attitude of Pentax (now Hoya)!
The Real Evil is indeed here. Just like the infamous SDM problem, what we Pentaxians could see that is Pentax never listens to the wide and loud complaints from the suffered Pentaxians public and has never tried some better efforts to resolve and eliminate the problems or just to be a bit more responsible (i.e., to act and react in a more positive way to their users/customers for the *real* issues)! Well, back to the AF problem, just say, how is it so difficult to make a smaller AF sensing area for more precise AF measurement and control, just like what their old Pentax MZ film AFSLRs in the 90s could do? How is it so difficult to include a "f/2.8" AF sensor (which just point to the outer part of the glass/optics of the lenses!)??
All these are just rather ridiculous after all! >:-{ At the end, the Pentaxians are hurt and it does no good to Pentax/Hoya, as I have been saying this for more than six years now - but still I do really feel sorry to see loyal Pentax users and supporters are still telling that they are suffering and being helpless/hopeless! >>:-((
So, after all these, the solution? Any?? My humble answer is that there is NONE, unless the Pentaxian switches, before Pentax/Hoya is really willing to do something and make the CHANGE! :-( (But Not Just Know how to Change the Colours of a Body!)
Read Also:-
Focus Calibrations for (Pentax) (D)SLR Bodies and Lenses
How AF Errors Can Decrease *Effective* Resolution?