07-25-2015, 11:35 PM
Field curvature plays no roll for PDAF. It is literally impossible for the AF sensor to sample a different image than the sensor would. If the lens' focal plane is shifted 300 microns at the edge focus point and you focus there, it will focus accurately for that position on the sensor.
AF adjustments correct error on the lens' mount location. While all lenses stop at the same position on the barrel at infinity or past or wherever the stop is, the focus may not reach the sensor correctly. The lens reports it is at position "1" and the camera will tell it to move to position "3800" to focus properly. If this is miscalibrated, 3800 may not truly be in focus. The AF adjustment is to ensure that "3800" is "3800."
Some older models have closer to 24 focus positions. Newer ones have thousands.
AF adjustments correct error on the lens' mount location. While all lenses stop at the same position on the barrel at infinity or past or wherever the stop is, the focus may not reach the sensor correctly. The lens reports it is at position "1" and the camera will tell it to move to position "3800" to focus properly. If this is miscalibrated, 3800 may not truly be in focus. The AF adjustment is to ensure that "3800" is "3800."
Some older models have closer to 24 focus positions. Newer ones have thousands.