Originally posted by Lev Yes you must be very careful shooting at 1.4 but I have one question regarding autofocus accuracy.
The answer is complicated, but is not related to accuracy. What you are seeing is the precision or lack of such due of the characteristics of the PDAF system. As you may be aware, the K-1 has two classes of AF sites, those sensitive to f/5.6 and those sensitive to f/2.8. By "sensitive", I mean the ability to detect being out-of-focus. If PDAF is sensitive to f/5.6, it means that the ability to detect OOF is the same looking through a f/1.4 aperture as through an aperture of f/5.6. The result is lower precision than with the same lens using one of the f/2.8 AF sites, but still not as good as might be possible if the K-1 offered sites sensitive to f/1.4*. Strangely, the narrower depth of field as well as more light to the sensor does not result in better focus precision (fewer images that are almost in focus). You will get a similar number of shots in perfect focus as might be available with an f/2.8 maximum aperture lens with the same subject. Of course, this behavior would be most obvious over the course of many exposures.
The reasons are related to how PDAF works and the optical limitations of split-image rangefinders. The full explanation requires perhaps a thousand words and many diagrams**. As you noted, CDAF in live view tends to have better observable precision, partially because its sensitivity is similar to that of the person viewing. When it falters, we simply don't notice. While the PDAF site sensitivity is a matter of physics and the sensor's design, there are other factors that make perfect precision difficult. Those include lens contrast, nature of the subject, and mechanical imprecision in the focus mechanism.
I might add that similar issues are traditionally present even when using manual focus. The stock focus screen on most dSLRs has a sensitivity of about f/4.0. The split image on aftermarket screens is sensitive to about f/1.4 at best and the highly regarded Canon S-type screen is sensitive to f/1.8 but is problematic at apertures narrower than f/2.8.
Steve
* Not offered on any commercial product that I am aware of.
** For a detailed explanation see Doug Kerr's excellent article on how PDAF works and its relationship to split-image rangefinders:
Link to PDF