I am a bit confusing about the term of Sharp Copy and Front/Back Focusing.
1)Is a non-sharp copy of a lens caused by front/back Focusing(inaccurate focus)?
caused by the internal optics of the lens not built precisely?
2)What make the lens lock focus? The Phase-detection on the body tells the lens that the subject is focused? If so, why does some lenses still have front/back focusing?