Almost one year ago, I bought my K-x. The first unit of mine was replaced owing to some problems, one of which was the obvious back focusing with the fast Pentax primes, including both the FA 43 and FA 31. Eventually, I got my another K-x unit only in another month, as the Blue Navy K-x was out of stock shortly (and so quickly) within the first week after the goods had arrived in Hong Kong.
Now, yet after another ten months or so, I discovered that my K-x, which *had* perfect focus *before* with my primes, say the FA 43 for example, *now* has significant back focus with the *same* lens and under perfect lighting condition, e.g., on a bright sunny day outdoor! :-(
What? You may ask my Pardon!? Yes, the AF accuracy of my K-x has disappeared and now focus has drifted backwards considerably after 10 months and several days!! :-o
Last week, when I first just shot outdoor for some portraitures on a clear bright day, when I got back to home, I found that most of my photos were out-of-focus when using larger apertures with two fast primes I used, namely, my FA 43 Limited and the DFA 100/2.8. I then decided to re-measurbate my K-x again. Yes, now it requires a +120 um adjustment into it (via the Debug mode, Tutorial Here) before everything can work back normally and as accurate as before!
Yet the luckier thing out of this most "unfortunate" case of mine is that the +120 um setting cures the back focusing problem for both my 43 and 100 prime, at least for close to middle distance objects (whilst for infinity and distant objects now my K-x will cause slight front focusing after the adjustment! >:-|)
Besides, I often said that if you want to confirm with the AF accuracy of your bod(ies), you should shoot real-life targets with a flat surface and with some good contrasty patterns on them but should be with irregular patterns. But, if you just wish to check for the amount of Back/Front Focusing more promptly on a "quick glance", then there is the 45 degree test chart and method outthere. Amongst all of the variants, I recommend to read this article and use this chart, by Bob Atkins, which I believe is also the originally created chart of this method.
On the other hand, the most widely spread "Nikon D70" test chart (I suppose) is not recommended by me as I found that its target line for doing the AF is just a bit thicker than ideal and thus may cause some errors, for either slight FF or BF, depending on the software algorithm of the AF system:-
http://focustestchart.com/chart.html
But afterall, what really puzzles me much in my this real case and experience is that I've lost almost the last bit of my confidence on the brand Pentax as the products they created are just without any durability and time longevity, even if they *did* work on the Day One! >:-\
Previous Stories:-
K-x' Durability/Production Quality is Just a Joke!
Self-Disassembled DA 21