Originally posted by Class A Nonsense!
There is a disadvantage associated with DSLR AF which is that if there is misalignment between the sensor and the AF system, there will be misfocus. But a properly calibrated camera does not have that problem.
Another potential problem is that the AF system sees something else than the sensor (due to colour temperature -> a problem the K-7 no longer has, or due to spherical aberration).
Not my words but I interpreted this based on the fact that drive motors (not sure of the ultrasonic) were steppers... there could be a "gap" , especially at large apertures or short throw lenses where either step is wrong (forward or back). Then the AF algorithm has a "best case" and "give up" mechanism. Of course this does also apply to "live" af as well. Add mechanical "slop" and my point was we are lucky it's even this good
I cannot find any logic in assuming that it is just a AF to sensor baseline adjustment is the only (or major) cause of AF adjustments. It can not explain lens to lens variations or same lens zoom variations in focus...there must be some other cause. The large aperture/larger baseline sensor is one solution to a problem unrelated to any "distance" problems.
color dependencies are known (Canon can misfocus in fluorescent lighting, Pentax in tungsten).
spherical abberaton problems w/ change in f stops seems to be more of an issue then any AF mechanical baseline errors..
Autofocus camera - US Patent 4890133 Description
Canon does run some open loop AF that is predictive. Which is why they can get the speed out of it.
http://doug.kerr.home.att.net/pumpkin/AF_accuracy.pdf Canon AF System Operation
When you half-press the shutter release (or the * button, if you've used the custom function to move focusing control there), the activated AF sensor "looks" at the image projected by the lens from two different directions (each line of pixels in the array looks from the opposite direction of the other) and identifies the phase difference of the light from each direction. In one "look," it calculates the distance and direction the lens must be moved to cancel the phase differences. It then commands the lens to move the appropriate distance and direction and stops. It does not "hunt" for a best focus, nor does it take a second look after the lens has moved (it is an "open loop" system).
If the starting point is so far out of focus that the sensor can't identify a phase difference, the camera racks the lens once forward and once backward to find a detectable difference. If it can't find a detectable difference during that motion, it stops.
Although the camera does not take a "second look" to see if the intended focus has been achieved, the lens does take a "second look" to ensure it has moved the direction and distance commanded by the camera (it is a "closed loop" system). This second look corrects for any slippage or backlash in the lens mechanism, and can often be detected as a small "correction" movement at the end of the longer initial movements.
Now to the OP I think a simple recalibration of the K20 is called for but my point was don't expect it perfect for all lenses all the time....... considering ALL the factors that go into AF-ing