This is not an issue with the camera (K5) as much as it is an issue with using AF with fast lenses. Simply put, the AF system, regardless of brand, is not up to the task when DOF is very thin. This includes catch-in-focus as well as focus confirm. Consider:
- AF precision is on the order of +/- a few millimeters. This is true regardless of how well-calibrated the camera is.
- DOF at portrait distances with a 50mm lens at f/1.2 at a reasonable final image size is also a matter of a few millimeters of acceptably sharp focus
- Actual desired focus point is difficult for the AF system to intuit. Translation...the camera has no way of knowing that you want the leading surface of the model's cheek to be the point of focus. Heck! The target zone for focus often covers several potentially good points for focus. The camera will choose the area that snaps to highest contrast first. So what if it is the trailing line of the lower eyelid!
The bottom line is that consistent fine focus with fast lenses is not a reasonable expectation from an AF system. The alternatives are a high quality aftermarket focus screen with focus aids or focus from live-view on the LCD using a magnifying loupe.
All that being said, I can reliably predict a chorus of examples from people who use catch-in-focus/focus confirm on a regular basis with fast lenses and/or for macro photography. A lot depends on the subject, the intended point of focus, the degree of enlargement, and the photographer's tolerance for missed focus. A good example would be doing catch-in-focus at 1:1 macro of an approaching insect, chances are that the AF system will do a credible job of firing just after the leading edge of the bug face passes through the plain of focus. The subject is high contrast and the point of focus is acceptable. Try the same thing panning in on a collection of white marbles in strong diffuse light.
Steve
(BTW...I regularly shoot my FA 77/1.8 wide open using AF. Works great as long as I don't push the envelope and am willing to accept the camera's judgement...)