Manually focusing the Canon 5D Mark III is somewhat problematic.
The non-replaceable focusing screen of the 5DM3 is not conducive to focusing an f/1.4 or f/2 lens (and perhaps not even f/2.8), which I quickly found out in focusing by eye— hit or miss accuracy, with mostly misses, using the Zeiss 100/2 and 35/1.4 and Leica 90/2 APO. Those images are not presented here because they were mostly out of focus enough to make a summary judgment on viability.
This page shows what can be expected using the Canon green dot focus assist (autofocus assist in manual mode).
Canon focus assist (green dot AF assist)
The Canon AF assist mode is partially brain-dead; it works ONLY with “chipped” lenses, which means Canon EF lenses, Zeiss ZE lenses and Tamron, Sigma, etc lenses designed and chipped for Canon EF mount.
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