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Old Aug 13, 2007, 5:25 PM   #1
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Hi all, if you have this setup and you confirm if I'm getting similar results to you or not?

I've had the 2x TC for a while but not been happy with the AF that I'm getting. I got it initially to shoot cricket however nearly every shot was back focused. I didn't do anything with it for a while and again went out to play this weekend and again got similar results. When the subject is closer it seems to be happier but not always accurate. If I MF the lens or actually what I do is focus on single shot and then tweak it to bring the focus forwards I get a nice result. I've noticed the same problem with the 2x TC on the Sigma 70-200mm f2.8 and I'm trying to get to the bottom of it.

Any ideas as everything seems fine when using my Sigma 1.4x TC on either lens. What have your experiences been?
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Old Aug 13, 2007, 8:56 PM   #2
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IMO it's not the 2xTC - What you have is the camera back focused slighly with the lens alone and when you put on the 2x it magnifies this defect

-> Post some pictures of a brick wall at 45 degrees angle in single-shot AF of all the combination of lens/1.4x/2x TCs. In AI-servo it's hard to tell what the camera is predicting forward/backward based on the movements it detected either from the subject or handshake as to the camera it's just motion...

Think about it, a TC alters the lens optically and is somewhat similar to putting a diopter in front of the lens, but the camera focusing can still compensate for this optical alteration. The TC is only different in that it transfer the communications betwen the lens and the body as it sit in between

When you adjust through the viewfinder it could means that the mirror is off as well through this path!
Does this happen on both your bodies ? :idea:
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Old Aug 14, 2007, 6:31 PM   #3
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Thanks NHL I'm going to confirm 100% that it is on both bodies but sure it was. I probably won't get a chance to do this until I get back from the US so will let you know how things go.
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Old Aug 14, 2007, 10:14 PM   #4
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Remember you have the same issue with the 70-200 f/2.8 combination as well... (with the same body???)
Mark1616 wrote:
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I've noticed the same problem with the 2x TC on the Sigma 70-200mm f2.8 and I'm trying to get to the bottom of it.

-> A TC/lens combo does not do the focusing. The AF phase detector below the mirror chamber looks through the resulting TC/lens combo and the camera based on this detector reading sends the appropriate commands back and forth to the lens so that the subject comes into focus @ the AF phase detector (and not neccessary the CMOS sensor as the mirror is in the path while focusing).

This mirror/sub-mirror assembly can be slighly out of alignement (i.e. not exactly @ 45 degree) and the whole deal is off. This mirror folds out of the way for every shot so it's one of the the most frequent moving part in the whole system (some camera can flip this mirror assembly 10 times a second!) so it's not very hard for it to be slighly off at rest with all this banging going on...

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