Originally posted by gm4life I think this thread explains what I witness once and a while with this lens. I thought it was that I was a crummy photographer or moving to much while open to F1.4... I still love the lens no doubt but I too get maybe 10 good and one bad... This AF thing could explain it and I double checked for front or back focus also... No adjustments were needed.
The point is that while we may expect PDAF to be 100% accurate every time we focus, the truth is it is simply not.
Do a test with the camera on a tripod with centre point AF only, and an unambiguous high contrast target. Between each attempt, make sure the lens is starting from a closest focus or infinity focus position (hand in front of the lens works well for this or use quick-shift). No need to look at the pictures, each time the camera achieves focus look at the barrel position distance scale, or take an iphone picture each time. They will all be very close but you will be lucky to achieve 10/10 identical ones.
This is why it is so important when doing a Autofocus Fine Adjustment to do 10 tests and adjust your AF/FA setting to the majority vote, and not rely on a single focus acquisition.
PDAF is nearly 40 year-old technology. Add in the variables of sensor versus AF-sensor misalignment (hence AF/FA menu), and something as simple as a speck of dust on the AF sensor and you have even more scope for inaccuracy.