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Next PZ lens test report: Tokina AF 50-135mm f/2.8 AT-X Pro DX
#10
Hi Wim:



Thank you very much for a very detailed answer.



Let me put my question in another way: Suppose that we have a lens which has focus shift problem when the lens is wide open and the problem is caused by the lens (e.g., wrong information communication with the camera); and we have a perfect DSLR camera which has perfectly aligned and calibrated AF sensors and a perfectly aligned OVF and a perfect focus screen so that what we see in the OVF is what we get. Then, mount this problematic lens on this perfect camera. Now let us do the following tests (the lens is kept wide open):



1. Manual focus the lens and judge the focusing state through the OVF by a "super" naked eye; until we see the image in the OVF is perfectly in focus then we press the shutter to take the image. Will focus shift happen in this case? (If focus shift happens then I will be surprized since I expect that "what we see is what we get".)



2. Manual focus the lens and judge the focusing state through the OVF by the "focus confirmation" function, e.g. the green dot in Nikon DSLRs. According to the article I cited in another post in this thread, the green dot in the OVF turns on only when the state in the figure 2 in that article is detected by the AF sensor. When the green dot turns on, press the shutter to take a picture. Will focus shift happen in this case?



According to your answer, for a given pre-focus state (e.g., the state in figure 1 in that article) the camera calculates the distance and the direction that the lens need move its elements to get precise focus (figure 2 in the article) and passes this information to the lens, then the lens just follows the order from the camera. If the lens misunderstands the information that the camera passes to it, then focus shift can occur. But in the above two cases I do not let the lens do AF, I do the focus job with my hand <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' /> So I guess here I am essentially asking "Can the wide-open focus-shift problem be solved by MF the lens?" (of course it is assumed that the shift is caused by the lens not by the camera.)



Best regards,

Frank
  


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Next PZ lens test report: Tokina AF 50-135mm f/2.8 AT-X Pro DX - by frank - 07-21-2012, 04:10 AM

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