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I have heard before that some say/ think that AF goes against best optical excellence. Could anybody give a very short general explanation what compromises optical quality in AF (without technical details)?
modern fast AF requires a small light focus group (or giant powerful motor i guess), this means changing the actual design of the lens and adding more different constraints to the cost/performance calculation (optical stabilization is similar in this regard). basically, the simplest way to make a high performance lens of given speed and focal length probably isn't conducive to fast AF, so instead a more complicated optical design is required. this means a higher price per performance ratio or cutting costs elsewhere (build, labor, quality control, etc).
the other issues are of course AF accuracy and calibration, which depend on the camera and lens together.