Originally posted by mikeavison Can anyone explain this to me. I had thought that all digital cameras use some property of the actual image (often contrast) to detect best focus. How could there be an offset required in such a situation?
What is discussed here is AF when using the viewfinder on a DSLR..... PDAF (Phase Detect). While using the viewfinder, the mirror is down, the shutter is closed and the sensor that captures the image is not being exposed to light. AF is done on an AF sensor that sits in the mirror box. Slight manufacturing tolerances can have an affect.
What you are describing is CDAF (contrast detect), which is done on the recording sensor itself. All mirrorless designs use this , as do Pentax DSLR cameras when in LV (live View) mode. Here the mirror is up, the first shutter curtain is open and the recording sensor is receiving light. The image, for framing is displayed on the rear lcd screen.
Any autofocus fine adjustment is not required in CDAF, and indeed there is no provision for it.