How is it that sometimes a lens will work fine on a camera body without adjustment yet others need some AF micro adjustment tweaking. I would imagine that the accuracy of the AF system with regards to front/back focus would be consistent across all lenses and be a body only issue. Yet it seems very common for different lenses to need different levels and directions of adjustment. What is it that could cause one lens to throw off the AF sensor but not another?
For example I have a good quality 100mm f/2.8 macro lens that I haven't noticed any focus issues with (though now I'm going to check again) and a cheap 70-210 f/4-5.6 zoom that needed micro adjustment. I would assume that a much sharper lens with shallower DoF would be more temperamental with AF, but that's not the case here it seems. How would the quality of the lens affect the AF module in the camera though? From what I understand of phase detect AF it should be the same regardless of the alignment and such of the lens, it should be just measuring the angle of light and be able to tell when something is in focus independently for the mechanics of the lens.
I think that this effect might be due to aberrations in the lens making the dynamics of the "circles of confusion" more complected than in a more high quality lens, resulting in poorer sharpness and offset AF.