Canon focus mystery

[Firestarter]

I think that Trowski hit the nail on the head (after the lensrentals.com article author). Misalignment between imaging sensor and AF sensor is the culprit.

[PaulM]

Consider also that there is a third path: from the main mirror to the focussing screen. If the screen is not in the right position then the viewfinder can appear out too. I believe Canon use thin shims to align that. I had an original EOS1 that had VF disagree with AF.


Paul

[Carlos Lindado]

Hey Trowski, thanks! That article definitively unveils many mysteries about phase-detection autofocus, it was the explanation I was looking for!


I think some bodies could just "tell" the lens to move its focusing group a certain distance, but don

[neuroanatomist]

Well, the article does explain why AFMA is necessary, but doesn't really address Bruce's fundamental mystery as I interpret it. I've seen something similar myself, as has scalesusa it seems.


For example, in my case:

• The 16-35mm f/2.8L II requires an adjustment on the 5DII that is 4 units negative relative to the adjustment on the 7D.
• The 70-200mm f/2.8L IS II requiresan adjustment on the 5DII that is 4 units negative relative to the adjustment on the 7D.
• The 85mm f/1.2L II requiresan adjustment on the 5DII that is 2 units positive relative to the adjustment on the 7D.
• The 100mm f/2.8L Macro IS requiresan adjustment on the 5DII that is 6 units positive relative to the adjustment on the 7D.
• The 24-105mm f/4L IS requires the same adjustment value on both my 7D and my 5DII.

<div>I had thought there would likely be systematic differences - a particular camera was x units off in one of 'true', and a particular lens was y units off of 'true', in theory if you knew those values for a given camera and lens you could predict what the AMFA should be for the combination of them. But in practice, that's not the case - each combination (at least with the bodies and lenses at my disposal) seems unique.</div>
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<div>So, from a theoretical standpoint is remains a mystery. From a pragmatic standpoint, it's irrelevant - I will just continue to do an AFMA for each body+lens combo I have...</div>