Hi Zoltan,
First of all, the very center of the pic is actually sharp. Look at the line of the side of the child's face in the very center. This focus plane would be just in front of the ear, and too far back to get good forcus at the eyes. if you superimpose an imaginary focusing screen grid over the pic, I think you'll see what I mean. the fact that part of the collar is in focus is probably just coincidental, since it's the same distance as the side of the face. The camera probably didn't focus on it. . .
I'm making a couple of assumptions. I'm assuming that you used center point focusing or select point with the center point selected, and I'm assuming that you either used AF or trusted the AF sensor to indicate proper focus. Even if these are not the case, what I have to say might still help.
With lenses that have very thin Depth of Field, as this one does wide open, you need to know how the focusing system works -- not just "press the shutter button half way and it focuses", but rather how big the focus sensor area is, where it's located, and what it's "looking" for, and to an extent, how it works. I will not state that this is definitive explanation of how the Pentax system works it's just some conclusions from some significant use, but I think it's pretty close. . .
1. The center AF sensor is approximately the size of the central ( ) circular area etched in the viewfinder.
2. The center AF sensor is the largest of the 11 in the SAFOX AF system. It is shaped like a "+" The surrounding eight focus sensor points are also cross-shaped, but more shaped like a "T" with the tail pointed in the general direction of the center of the frame, and are a bit smaller. The two outermost sensors are shaped like an "l", and are line sensors. The center point is located at approximately the central circular etching, but might be displaced slightly due to manufacturing tolerances and/or some limited shifting from impact during shipping or use.
3. All AF sensors detect contrast borders across their areas, and are most sensitive to such borders at right angles to the sensor "lines", so cross-type sensors will look for contrast borders within their areas, but will be most sensitive to higher contrast, and borders that are closest to vertical or horizontal. The line sensors will be most sensitive to contrast borders that are closest to horizontal. The center sensor doesn't much care between horizontal or vertical since the two perpendicular "lines" are of about equal length, but the surrounding cross-type sensors might show some bias from their actual shape and positioning. My guess is that they look for highest contrast first, then, it prioritizes to lower contrast borders that are closer to either the vertical or horizontal planes relative to the viewfinder orientation. Remember that anything within the AF sensor's area is fair game, so make sure that the area that you want to focus on is the dominant one, either by degree of contrast, orientation to vert or horiz, or both.
At this point, let me state that I realize that DSLR AF is usually called Phase Detection as opposed to Contrast Detection, but this terminology has more to do with how the AF system controls the AF motor than how it detects focus. . .this photography stuff can be confusing. . .
Your job as the photographer is to make the camera's AF system work to its greatest potential and accuracy.The thinness of the DOF for a wide aperture lens has been discussed, and I won't repeat it. Also the benefits and detractions of "focus and recompose" has been argued ad ifinitum elsewhere, so I'll defer to those with more exacting requirements than I. Others will argue that the only way is MF, but I can't do it very well, especially in low light, so I'll just discuss AF. . .
What I'd try first is focus and recompose. This is the easiest to learn, the least fiddly and potentially confusing while shooting, and is good enough for most shooters and situations. Keep in mind that what you need to concentrate on is getting the area you want in focus in the central circular etching, actuate AF, wait for a lock (indicated by the steadily lit green hexagon, not the little red light that flashes on), recompose the frame while holding the shutter at half press, then fully depress to take the shot you want. Realize that closer distances and wider apertures both make DOF thinner (there are other factors, but these are probably the most important to remember), so if the DOF is too thin to get the eyes in focus, one strategy would be to back off, take the shot, then crop to get the composition that you want. The alternative is to use a smaller aperture, but in available light shooting, this will make for longer shutter speeds which can make camera shake more of an issue, you have to decide.
You might find this technique doesn't work for you -- an alternative is to first compose the frame, then select the focus point you want the AF system to use by manually selecting your focus point. This is going to be more accurate technically, but has the downsides of potentially being slower, more fiddly and confusing, and uses sensor areas that might be a bit less sensitive and/or accurate. You really can only decide by trying them both. . .
Remember to try to get a high contrast border within the area of the focus sensor, and that they prefer borders that are vertical or horizontal. You might try the border between the iris and the white of the eye, or the eyelid and the white as focusing points when you're at close shooting distances like this. If you're shooting something that has less contrast, and the AF has a problem locking, it might help to twist the camera slightly to get the border to more closely align to the focus sensor orientation (either straight v or h), then tilt the camera back to straight, once you've locked focus.
Sorry for the wordy post -- hope it might help. . .If you already knew this stuff, then you can just disregard it. . .
Scott
