Originally posted by pacerr I have a hard time using that video as an explanation as to WHY multi-point AF is better than single point. The video maintains a fixed view of the subject area which makes tracking the nearest element of the subject as it crosses the frame seem logical. However, were I making that shot I'd likely be panning to keep the subject at about the same point of composition within the frame such that a single AF point might be the logical point of focus for the subject.
It only makes sense to me if one assumes that the nearest point in focus within the AF array is always the desired point of focus in the scene. And that would only work well if the camera was capable of processing speed and lens reaction time suitable for accurately predicting a continuous focus solution on any moving target.
There are two functions from what I can see. The extended area af will start with the selected point and keep focus on the subject. I suspect that it is by a few different means, using a contrast shape or color, and some rate algorithm to determine movement closer or farther. It does that reasonably well, not prescient but consistent. It has trouble if there isn't anything of notable contrast to work with due to either low light or simple low contrast. A simple test is to focus the initial point on a subject about the size of a focus point, then pan slightly within the extended area points and it will follow, you can see the other points twinkle as they are used. The K-5 would select the highest contrast within the points.
This would be the foundation of a tracking system, and if the focus points are accurate and there is adequate contrast, it works well. The K-5II had this functionality from what I understand.
The K-3 has another function which adds a bias to keeping focus on the initial subject focus distance. In the simple test above, if you pan the object past the extended area points it will refocus on something closer or further away. The hold setting of low, medium or high will bias a tendency to not refocus. It seems that the longer you have a subject in focus the more the bias towards not refocusing. If you focus on something, holding the AF or half shutter, then immediately pan to something else it won't hold. This is common usage to find the point on which you want to focus. But if you focus, maintaining AF or half shutter button, hold it there for a bit then move the refocus will be delayed.
The video example indeed could be easily replicated by panning technique, but for example tracking a bird in flight. It is very hard to keep them accurately lined up for single point focus, so extended area works nicely. If you lose it as it changes direction or something comes between you and the subject, the hold allows you to keep within the focus distance without the lens hunting back and forth and you losing the shot altogether.
It isn't perfect, I think the next step is to integrate recognizable shapes such as eyes, face or head profiles. The Pentax isn't smart enough to pick out the subject you want to follow (at least in PDAF). But it is predictable. In a given scene, or a similar scene it will do the same thing, which allows me the photographer to get to know the limitations and strengths and work with them for good results. That was not the case with the K-5, it was unpredictable. I experienced that the other day, with my K-3 in for repairs. Perfect light, late afternoon through a high thin cloud generating warm high contrast light, moments we wait for. Swans not too far away doing what swans do, and there would be 3 shots in perfect focus, two not, 4 good, a couple not. The K-3 in similar situations produced one shot after another focused just right.
A technique that K-3 users will learn is to touch and release the focus button to reset the hold.
The extended area focus works for focus and re-frame without the angle changes losing focus.