Understanding Auto Focus in D2 series

Discussion in 'General Technical Discussion' started by Rich Gibson, Aug 24, 2005.

  1. I've mentioned this before but I wanted to bring you up to speed on an excellent article at the Nikonians web site.

    http://www.nikonians.org/cgi-bin/dc...973&forum=DCForumID71&omm=1&viewmode=threaded

    Darrell Young (digital darrell) has finished the article, replete with illustrations, menus, etc. on how to understand and use Nikon's multi-faceted focus system. He even offers suggested settings for various type shooting. It is an essential read for anyone who owns or plans to own a D2 series camera.

    Rich
     
  2. general

    general

    Apr 30, 2005
    Nebraska
    Hurrah for Digital Darrell

    I have followed the development of Darrell's article from the early stages and I, for one, really appreciate the effort he has put into this. It is an excellent exposition of a complex subject.
     
  3. Killer Link, thanks for posting this!

    _/oe
     
  4. cknight

    cknight

    663
    May 2, 2005
    Madison, AL
    Would it be safe to say that most of these principles would apply to the D70's focus mechanism too (except for the features the D70 doesn't have - such as focus lock)?
    The main thing I'm interested in is the operation of the Single/Continuous servo modes, and Single/Dynamic focus area modes.

    In dynamic mode, the autofocus can choose to focus on something not in your target focus area if something else is more contrasty. Does this change what situations you would use dynamic mode for? I'm thinking flying birds, but woud be interested in other situations too.

    I guess the firmware's assumption is that your target will be the most contrasty subject in the viewfinder, so if an area of greater contrast shows up, then your subject moved. That may be true in some cases, but not all.
     
  5. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    I'm afraid I do not understand Darrel's suggestions on a4 setting, particularly "Custom setting a4 ... is only concerned with WHAT it [camera] is focused on". From Darrel's description, Nikon manual and experiments a4 works controlling camera refocusing in cases when the subject "jumps" back or forth, or any "foreign" object appears in front of the subject of focus, obstructing the view: "This option controls whether the camera immediately adjusts focus to track a subject when the distance to the subject changes drastically." See also page 185 of the manual. Delay caused by setting a4 is substantial, and camera IMHO has no means to differentiate between sudden move/turn of the subject and another subject coming into view. Try yourself - focus on a car driving about 30MPH so, that it will be framed inside the central circle of the viewfinder and watch how your camera will not refocus on the car moving in the opposite direction. The lag seems to be about 1/3 of a second. So, if there are little chances that the subject I'm after can be obstructed, I always switch a4 off.

    The tricky part is that when you follow the subject with the camera, and do it fast, having fairly contrasty background (small aperture adds to that) - a4 off can force camera to refocus on the background! Here closest subject priority is helpful.

    For this case I use Pattern 2, Center 1 or Center 2 (p. 184 on D2X manual), depending on the primary direction of the moving subject, with closest subject priority - if the subject acts solely and focus area on the subject is smaller then 1/3 of the frame in longer dimension. If the chances of obstruction are great - here a4 comes into play, sometimes together with switching closest subject priority off.

    The whole "erratic subject behavior" can be fully applied to how the photographer follows the subject. Long lenses, hand-helded - that can be the case for "erratic subject behavior" even if the subject stays still :)

    "Obscuring" can be referred to contrasty objects in the background. a4 "on" prevents from focusing on background, but so does closest subject priority.
     
  6. Thank you very much Rich for providing this link, very useful informations. It will help me get better pictures.
     
  7. Thanks Iliah for your comments. I've seen this article previously but will revisit it. I'm curious to try these suggestions to see what results I get when trying to track a flying bird with a busy/contrasty background. For me, most of the time I do not have issues but occasionally I lose focus while tracking. Sometimes I can't get the camera to even lock focus on the target when it's a smaller bird. My settings are still set at default.
     
  8. mcampos

    mcampos

    96
    Apr 14, 2005
    Norwalk, CA
    From my experience that article has some flaws that go against what Nikon's upper level technicians recommend, specially the A4 setting. I would take its recommendations carefully.
     
  9. Darrell mentioned while writing the article he was on the phone repeatedly with high level technical folks at Nikon getting the official view. Which items specifically do you consider flaws?

    Rich
     
  10. mcampos

    mcampos

    96
    Apr 14, 2005
    Norwalk, CA
    Rich,

    During the time I was having problems with my D2X I spent a considerable amount of time working with a level 3 technician (I don't know how many levels there are). His recommendation was to turn off A4, he added that doing so seemed to give better results and that Nikon USA was waiting for an answer to a request for better understanding of the implications of this setting from Japan. He lead me to believe that the way the setting was designed was as stated in the literature, but that something was causing it to behave differently. I posted one of the responses from this technician on the thread discussing this article.

    Also a lot of people disagreed with the recommendation to setup the shutter release to focus priority in AF-C. I agree that this recommendation would lead to more problems than it could potentially correct.
     
  11. JeffKohn

    JeffKohn

    Apr 21, 2005
    Houston, TX
    I definitely disagree with that recommendation. If you focus with AF-ON and not the shutter release, it makes focus/recompose pretty much impossible.
     
  12. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    IMHO AF-ON is one of the most useful camera features. Why should I focus, recompose, shoot, and then - refocus for the same scene, just because I want a slightly different composition? Having separate AF and shutter release is muc like manual focusing, which makes a lot of sense to me.
     
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