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Next PZ lens test report: Tokina AF 50-135mm f/2.8 AT-X Pro DX
#11
Hi Frank,



I'm not Wim, but I enjoy the discussion and learn <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' />...



[quote name='Frank' timestamp='1342843848' post='19506']

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".)

[/quote]



Firstly, I assume that you use the term "focus shift" not as the definition of one of the optical aberrations, but simply as "shifting of focus" or "missing focus"... Based on that, I think the wrong focus in the above case would be resulting because of the misaligned camera parts (considering the distance that the light travels until the OVF and the distance between the respective rear element until the image sensor). I guess the precision is the key word here... Especially if one checks 36mp images on 100% views.



Quote: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.)



For the 1. case above MF or AF lens used in M mode does not matter, because you don't use the focus confirmation.



And regarding the green dot: The precision of the focus confirmation dot can be a problem for some fast MF wide angle lenses. For example, if you take the Zeiss 35mm f/1.4 and focus on the closest object, the focus confirmation dot might give a wrong info. The dof for this lens at wide open for that distance is very narrow.
  


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Next PZ lens test report: Tokina AF 50-135mm f/2.8 AT-X Pro DX - by PuxaVida - 07-23-2012, 07:09 AM

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