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Front/back focusing essay on m4/3?

Discussion in 'Open Discussion' started by Cerebus2, Jan 21, 2011.

  1. Cerebus2

    Cerebus2 Mu-43 Rookie

    21
    Oct 12, 2010
    Luminous Landscapes posted a guest essay about sources of front/back focusing errors on digital cameras:

    Are your pictures out of focus?

    The question I have is that with live-view cameras like m4/3, can this problem even exist? It would seem to me that the use of contrast focus at the image sensor would counter most (all?) sources of focus error in other systems. This in turn would help explain the amazing results we see with many lenses on these cameras.

    Of course, I could be wrong.

    -- C
     
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  2. bogolisk

    bogolisk Mu-43 Rookie

    19
    Sep 30, 2010
    AFAIK, no! front-back focus is an artifact of the dual sensor system: the imaging sensor and the focus sensor don't "see" the same thing. CDAF (or the Fuji's single sensor PDAF) use a single sensor.
     
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  3. Brian S

    Brian S Mu-43 Top Veteran

    714
    Apr 11, 2009
    The only way it could happen on a liveview camera: the lens is focused with the lens aperture wide-open and then stopped down for the exposure after the focus is set. Focus shift can occur when a lens is stopped down.
     
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  4. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
    I would not take everything you read on LL too seriously.
     
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  5. Brian S

    Brian S Mu-43 Top Veteran

    714
    Apr 11, 2009
    On a DSLR, the camera performs autofocus with the lens wide-open. So if you have an F1.4 lens, the AF mechanism will focus with the lens at F1.4 no matter what F-Stop you are actually using. If you use F4 for the exposure, the focus will be behind the point that the AF mechanism focused on. That is focus shift, and is just a property of a lens.

    [​IMG]

    Focus shift is given in these old Pop Photo tests. A shift of 0.04mm (spherical) means the focus will be ~0.5" behind a point of focus set to 36".
     
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  6. joele

    joele Mu-43 Regular

    161
    Dec 12, 2010
    Melbourne, Australia
    This the one advantage of how the manual lenses stop down on our u4/3 bodies.. It does make it harder as it gets darker in the EVF but it is like a permanent DOF preview and it also avoids focus shift errors which on lenses like my Pentax 50/1.2 where pretty annoying on Pentax bodies.. I wonder if it is also a factor that would have made the Voigtlander 25/0.95 cheaper/easier to design as it is not really an issue if you set your aperture and then focus..