Nikon 51 point AF system in D300

Discussion in 'Nikon' started by aniramca, Jan 1, 2008.

  1. aniramca

    aniramca Guest

    I read some info about the 51 point AF system for the new Nikon D300.
    The dpreview showed a graphical presentation on the locations of those
    51 points within the targeted digital frame (approximately covers 2/3
    of the square, centered in the middle of the frame). Could someone
    provide a little more detail about this 51 point system?. I may be a
    little confused about this concept against the exposure metering in
    some old digital cameras.... averaging or spot metering. I guess that
    this is not about the light exposures, but instead it is about the
    distance measurements from the object(s) to the sensor plate. If the
    later is correct, then how do they manage to get a razor sharp picture
    if you have to average the distances over 51 spot locations? Or, did
    the camera actually take average distances from those 51 locations? I
    guess you cannot have 51 different focused distances to the sensor and
    each measured distance will be registered into a single 2D plate, can
    you? Or, I may miss something completely and more fundamental here? I
    think that taking average distances over 51 points will not definitely
    work to produce a razor sharp image on the main focus/target area,
    unless each of those 51 points have identical distances (or close to
    identical distances) to the sensor.
    Thanks for discussion and comments.
     
    aniramca, Jan 1, 2008
    #1
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  2. aniramca

    Jim Guest

    I can't help with the D300. However, my D70 has several points, but only
    one is in
    use at any given time. The only connection between AE and AF is that some
    cameras use
    the distance to the focus point as a guide to set the exposure.
    Jim
     
    Jim, Jan 1, 2008
    #2
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  3. aniramca

    Woody Guest

    More likely that the uP reads the various distances, decides which are
    the near ones (and thus likely the subject(s)) and adjusts the focus
    point for those, possibly adjusting exposure by aperature at the same
    time to ensure sufficient depth of field to cover them all.
     
    Woody, Jan 1, 2008
    #3
  4. aniramca

    Ben Micklem Guest

    ,
    The sensor points are not magic 'rangefinders' that output distances. They
    are phase detection sensors. Some are 'cross type'- i.e they detect in two
    planes at the same time.

    See the following pages on Nikon's site:

    http://preview.tinyurl.com/2mnrns
    http://preview.tinyurl.com/36dj2z

    There are many different focus modes on the D300:

    € Single point AF
    € Dynamic Area AF [9 points, 21 points, 51 points, 51 points (3D-tracking)]
    € Automatic-area AF

    I think in dynamic area mode, you can set whether it biases the choice to
    closer subjects, or not. It will also automatically start focus tracking
    (predicting where a subject will be, and focussing there before the subject
    reaches that point). Combine all these options with AF-S and AF-C modes, and
    you're looking at quite a few combinations of settings.

    In Live View mode, it will use contrast detection AF on any desired point in
    the frame (i.e. change the focus, see if this increases contrast, if not, go
    the other way, etc.). All other AF modes won't work, as the focus sensors
    are in the bottom of the mirror box, and they get their image from a
    secondary mirror that reflects light from the middle of the frame (that has
    passed through the semi-silvered primary mirror). As the mirror is up during
    live view, this can't work. I think there is an Olympus live view method
    where the live view comes from a second colour CCD in the prism housing, and
    so the mirror is down.

    HTH,

    Ben
     
    Ben Micklem, Jan 3, 2008
    #4
  5. aniramca

    Guest Guest

    the nikon d3/d300 live view has two modes. in handheld mode, the
    mirror will drop and autofocus will use the usual 51 point focus
    sensors, then the mirror goes back up to resume live view. in tripod
    mode, contrast detection is used off the sensor itself and the mirror
    does not drop to focus, with the focus point placed anywhere in the
    frame.
     
    Guest, Jan 4, 2008
    #5
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