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05-06-2011, 12:10 PM   #1
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AF and viewfinder optical paths in the 645D Question.

I have the FA and A vesions of the 120mm macro. I did a head-to head test and found some confusing results, which someone more knowledgable may be able to explain. I did a focus test at an object distance of 15 ft with both lenses wide open at f/4. The AF focus is spot on, but the A version front focuses by about 6 inches, even using focus assit, which I assume uses the same path as AF itself.
Anyone know how this could be?

05-06-2011, 01:58 PM   #2
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I don't understand? Both pictures are sharp, but the lens barrel markings are different? Or the AF assist is saying focus is at a different point? The AF assist is a little sloppy with manual focus lenses. Do you confirm the focus point with the ground glass? AF lenses can fine tune near the focus point with the AF motor--my DFA 55m does that. I imagine a manual focus lens having no link to an AF motor guesses more with focus assist. BTW, how repeatable is this and does it matter if you are focusing from infinity or the minimum focus distance? I find it makes a difference with my 105mm 67 lens, but I also use my ground glass to confirm focus.

Does that make sense?

Does anyone know if the FA lenses have a distance encoder? I believe distance encoders impact AF accuracy.
05-06-2011, 02:20 PM   #3
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I used a yardstick fifteen ft from the camera and at a slight angle and focused on the midpoint (16"). both pictures are sharp but the A version is focused at the 10" mark, whereas the FA is at 16", as it should be. I used the ground glass and focus assist with the A. I tried this test since I had seen evidence of front focus in some other shots with this lens. I do not see focus error to this degree with a 67 90mm, so the problem appears to be this with this lens and not MF in general. I usually iterate around the focus point a few times when manual focusing, stopping as I approach from infinity.
05-06-2011, 02:52 PM   #4
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To be honest, a 6" discrepancy over 15' is nothing to worry about. I don't think your focus assist can be more accurate than that with those condition and if the rule is at an angle, the distance is less than 6". Most likely the 67 90mm lens is performing better because of the larger aperture--it is a f/2.8 I believe.

05-06-2011, 03:29 PM   #5
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Not worried, especially since the FA is perfect, I'm just trying to understand why the focus assist doesn't appear to be as accurate as the AF since they must use the same sensors.
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