AFMA 5D3 and AF observations

Started Jul 5, 2014 | Discussions thread
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larry Contributing Member • Posts: 889
AFMA 5D3 and AF observations

I recently purchased a 5D3 to replace my 1Ds.  On the 1Ds, my 24-70 had incorrect focus at 24mm and distant objects (landscape trees, astro).  So I was quite overjoyed at the increased sharpness I've seen with Auto Focus Micro Adjustment.

I originally tried to AF with a black line on a white sheet of paper, illuminated with a high-end LED flashlight (a Zebralight SC52).  I quickly discovered that the focus was still off.

I then used direct evening sunlight at a distance of 30 feet.  And a week later, used a tree about 500 feet away.  Here are the results:

24-70 at w/t, f2.8, 30 feet: -13 and -11

24-70 at w/t, f2.8, 500 feet: -6 and -9

70-200 at w/t, f2.8, 30 feet: -6 and -8

70-200 at w/t, f2.8, 500 feet: 0 and -9

400 f5.6 at 30 feet, +10

400 f5.6 at 500 feet, +11

560 f8 (1.4x) at 30 feet, +1

560 f8 (1.4x) at 500 feet, +7

These results are with using the half-sized AF point in the center of the array.

I also wanted to find out how large a subject's details needed to be for the AF to focus.  With a pencil lead's-width (about 1/2 mm) black inkjet line on matte paper, the 24-70 needed the line to be about 2.8 pixels in height in the resulting image, for focus to be achieved.  The 70-200 and the 400 5.6 needed it around 3 pixels in height, and the 400 with the 1.4x tele on it needed about 4 pixels worth of thickness to the black line.  I would make a wild guess that the pixel sizes in the AF module are 1 to 2 times the size of the pixels in the 5D3's imaging chip.  I found, much to my great surprise, that the inner AF box in the center of the center AF box corresponded almost exactly to where the line needed to be, for the AF to lock on.

AF was achieved just as consistently with the line in horizontal and vertical orientations.

Does anyone have results that vary with subject distance?

Canon EOS 5D Mark III Canon EOS-1Ds
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