Originally posted by biz-engineer I would have thought that focus error comes from a difference between the AF optical path and the direct path leading to a focus error regardless of the focal length.
The subject is complex, but it enough to say that the issue with PDAF front/back focus is not related to physical distances on the optical path
per se. I would provide a link, but have not found a reference that provides a direct and concise explanation. The best I might do is to make the comparison to a classic split-image rangefinder* where instead of a clean split of a clear line, one sees a fuzzy line darker on one side and where the fuzziness and asymmetry is due to optical aberration and/or alignment defect.
Steve
* PDAF is essentially the same, though on a very small scale and using a linear array of photodetectors rather than the eye to evaluate.