Originally posted by slip some where front focusing, some back focusing and others (be it lower rate) focus perfectly.
Originally posted by slip yes but they don't seem to be consistent results
Some front, some back, some just right? It sounds like you are getting a real-world lesson in precision* (or lack thereof). What you are seeing is what you should expect when the camera is having a tough time of it or when your standards are higher than what the system can produce.
The base precision at moderate distance with a sharp f/2.8 lens using the center focus point on a high contrast (dark black bordering bright white) subject is pretty good. Increase the distance, soften the lens, and/or substitute a low contrast subject and attaining focus becomes more and more a guessing game for the camera. At worst, it simply gives up. At about 150m with a 200mm lens, my K-3 can be off +/- 7-10m easily; assuming, of course, that the focus mechanism for the lens is able to move in that small an increment.
That is why AF fine adjustment should be done at moderate distance with a high contrast target and should involve multiple focus attempts. (I usually suggest a minimum of 10 with 20 being more acceptable...half focusing from near and half from far.)
Steve
* The tendency in everyday talk is to treat precision and accuracy as meaning the same thing, but they aren't. A measurement system can be accurately calibrated, but provide unacceptable measurements most of the time due to poor precision (the probability of getting the same answer twice in a row).