Originally posted by TXPentaxK50 So are most lenses slightly out of focus and they do in fact need adjusting?
Many lenses may benefit from AF fine adjustment*, but it is not the lens that is out of failing
per se and it is not the lens that is being adjusted. The same lens may be fully capable of "bang-on" manual focus or CDAF focus in live view. The full answer is fairly long and complex, but the short answer is that the PDAF system accepts an ambiguous signal as being not-out-of-focus when it still is. The AF fine adjust applies a numeric bias such that that the PDAF system tells the focus motor to stop a little bit either side of that ambiguous point.
Steve
* I hesitate you use the word "most" because that has not been my personal experience with the AF lenses on my shelf. Of the seven I own, only one has required a fine-adjust setting. If one is seeing consistently soft results across the board with all lenses in both live view and with the optical viewfinder, the problem may lie somewhere other than focus.