« on: April 22, 2014, 03:33:50 PM »
To further add, my 1Ds3 and 1Dx never miss at f/1.4 on the outter points. Never. Always exact. No variability. Nails it everytime and that's even without AFMA.
it is physical impossibility because the AF measurement angle accuracy in Canons AF is F-2,8 ( eg.3,4)
That's not correct.
The specified precision is within the depth of focus at the max aperture of the lens for a standard precision AF point, and within 0.33 depth of focus (0.5 for some models) at the max aperture of the lens for a high precision AF point.
I discussed this issue with Chuck Westfall (Canon USA's technical mouthpiece), and this is part of his response:
"The fact that the AF points are functional with apertures as small as f/5.6, f/4 or f/2.8 respectively depending on the camera model and AF point under discussion does not imply that their measuring precision is limited to the depth of focus at those apertures. The AF detection system has the capability of calculating depth of focus based on the maximum aperture of the lens, whatever it happens to be."
I've been doing some reading and came across a pertinent point. Your post above relates specifically to high-precision AF points. The outer points are not high-precision, unless I misunderstand him here: "...one of the consequences of the TTL-SIR AF system is that except for the center point, AF precision is not proportional to the maximum aperture of the lens in use."
Of course, that's a 70D-centric dialog. Maybe some of the outer 1Dx/5D3 cross points are included.