Originally posted by BigMackCam Only the central phase-detect AF point is accurate to f/2.8 sensitivity - the others are, I believe, at f/5.6
This ^ ^ ^
...though the issue is not accuracy,
per se, but the ability to detect an out-of-focus state. The f/5.6 points will detect OOF with the same precision at say f/2* as at f/5.6. Similarly, an f/2.8 point will detect OOF with the same precision at f/1.4 as at f/2.8. That being said, on multiple attempts using identical technique both types will average to the
same point of focus.
The implications when doing AF fine adjust are worth considering:
- Don't compare the f/5.6 points to the f/2.8 points unless you are willing to do a statistical sampling
- When doing an adjustment, don't be satisfied with a single confirmation attempt
- When evaluating AF, do so de-focused from near and far lock on the focus ring. The system's notion of acceptable focus is a range and may vary depending on the start point.** Again, multiple attempts will yield better results.
Steve
* similar rate of missed focus at any aperture f/5.6 and faster
** This is easily demonstrated with a traditional fast manual focus prime using the focus confirmation feature. The green hexagon stays lit even as you move the focus ring past the initial point of focus to a point where it goes out. The true in-focus result for the subject is somewhere in between.