But whenever the environment is not under control, a well-working continous auto focus can be really, really helpful.

Yet all the recent implementations of continous auto focus in cameras using contrast based focusing suck - they just don't work good enough to be useful. They are either to slow or do too much visible "focus hunting".

I understand that any focusing based on contrast only will require some sort of "hunting", because the direction in which to adjust the focus plane cannot be predicted based on contrast only.

But continous focusing is relevant mostly for video (not for still images), and the resolution of the sensors is usually much much higher than the resolution of the recorded video. So it seems like a very obvious idea to adjust the focus plane only by the smallest possible amount (according to the step motor of the lens), one step back, two steps forward, one step back, and if properly sized, these steps are small enough that the change of the focus plane is not perceivable in the (relatively low) resolution of the recorded video, while it is still big enough to yield a significantly different average contrast in the (relatively high) resolution of the sensor.

I am pretty convinced that my old Sanyo hybrid camera did precisely that: With just one sensor chip and no phase detection, it could only base focusing on contrast. While shooting video, you could hear a very faint noise that was indicating the lens was focusing back and forth just a little - not as much as to be visible in its 720p output file.

Now I wonder: Why don't they just implement this simple method in current cameras? Why do they follow this primitive approach of "accepting some variation in contrast without moving the focus plane at all, then suddenly hunt focus from scratch"?