Focus distance

Discussion in 'Digital SLR' started by Warren, Dec 2, 2007.

  1. Warren

    Warren Guest

    I have no idea how autofocus systems work, however I was wondering if the
    process involves measuring the distance to the point of focus. If so, do
    any cameras record this in EXIF data.

    Regards

    Warren
     
    Warren, Dec 2, 2007
    #1
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  2. Warren

    Paul J Gans Guest

    Most autofocus by getting the contrast to a maximum in
    the small focus area of the screen.
     
    Paul J Gans, Dec 2, 2007
    #2
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  3. Warren

    Guest Guest

    nikon dslrs record it, but it is not an exact distance measurement. i
    don't know about other cameras.
     
    Guest, Dec 2, 2007
    #3
  4. Warren

    frederick Guest

    AFAIK, the focus distance reported back from the lens to the camera body
    is from an electrical slider over contacts measuring several (8?)
    positions of the focus ring, so is not accurate (good enough for use in
    flash exposure). There seems to be standard exif data for focus
    distance, but cameras that I've used leave the field blank even when "D"
    type lenses are used.
     
    frederick, Dec 2, 2007
    #4
  5. That is the method used in compact cameras (vulgo P&S).

    DSLRs use a beam splitter and phase detection, similar to an old split-image
    distance measuring device (although dSRLs only align the images and don't
    care about the actual distance information itself).
    Only if the dSLR has a live preview (some new models do) then live preview
    requires the mirror to be locked up and therefore phase detection cannot be
    used. Only in that case the dSLR will revert to the slower contrast
    analysis.

    jue
     
    Jürgen Exner, Dec 2, 2007
    #5
  6. Warren

    Guest Guest

    what exif viewer? nikon puts it in there (which i assume you have
    since you refer to it as a 'd' lens). however, as you say, it is a
    rough approximation.
     
    Guest, Dec 2, 2007
    #6
  7. Warren

    Paul Furman Guest

    Isn't that what it exists for? ...flash & 3D matrix metering... my
    understanding is it's not accurate enough to give a meaningful distance
    to humans, for a wideangle you see something like this at roughly equal
    intervals:
    1-1.25 foot
    1.25-3 foot
    3 foot to infinity

    even if you break that down into 3x the detail, the last segment looks like:
    10 foot to infinity
     
    Paul Furman, Dec 2, 2007
    #7
  8. That's the sensor image contrast method used by at least most of
    today's P&S, abd very few of today's DLSRs (because of the mirror). I
    think very few P&S are as primitive as that these days. Most by
    default use several areas of focus and average logarithmically between
    them, although you can switch to a small focus area if you want.
     
    Chris Malcolm, Dec 2, 2007
    #8
  9. In DSLRs, they use a digital version of the split screen.
    No, but some, not all lenses report the distance.
    This is used in certain cases for correct flash exposure.
    Canon does no longer --- too inaccurate, according to them.

    Which is kind of unfortunate, since that information can be
    used to combat barrel/pincushion distortion.

    -Wolfgang
     
    Wolfgang Weisselberg, Dec 3, 2007
    #9
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