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AF Under Stress

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  • AF Under Stress

    I own a G1 and in two days I will be overseas, so I have no much time to make tests.

    My question is what are the limits of the Face Detect mode. For example concerning the size of that face relatively to the whole frame, or the position near the borders, whitin which the camera will not be able to face detect and focus accordingly.

    The second questions refers to the size of the focusing frame. How the autofocusing will behave if I enlarge the frame to the maximum size. Will it focus in a sort of "center weight", or the neares object inside the frame, even if it is close to the border (within the border), or what ?

    Thanks in advance,
    tv/telephone/old toaster. And a computer !

  • #2
    Re: AF Under Stress

    I've never tried maximum size AF frame, I see no point in that.
    I use almost always smallest, or one size up, no larger.
    Otherwise I can't get a focus target in the box.

    Face detect seems to work to within 1/8 of the screen dimension from each edge.
    But I turned it off after it recognised books on a bookshelf behind my subject as a face and prioritised it for focus.

    Too damn clever, or not clever enough?
    Either way it's off now on my camera.


    • #3
      Re: AF Under Stress

      Ok, I have found more info: For a 14mm lens it detects a face with the green screen rectangle sign at 2,70m, which is very good given the focal length. Beyond that, it seems to detect the face, but from the camera screen display I cannot be very sure.

      As for the 1/8 detection zone, let's do some math, since fractions are a matrix area in my brain where I am out of focus. So I need an easier-for-kids formulation. So, if you like, go with me back to the kindergarden.

      Let's supose all the screen frame is a "negative" of 40 x 30mm. Or that 40mm X 30 will be the final small pic. 1/8 from each sided of the 40 mm = (40:8) x 2 = 10mm eaten at both sides = 30mm effective frame for horizontal face detection.

      For vertical face detection let's do the same formula (30:8)x 2 = 7.5 waist. 30 minus 7.5 = 22.5mm true vertical effective area of face detection.

      Therefore our former ("negative") 40 x 30 frame in our eyefinder, gives a true face detection frame of 30 x 22.5mm. Hmmmm, not more usefull either.

      So far, the formula is not very usefull, therefore let's try with percentages, which may be easier to imagine. What confuses me is the 1/8 from each side, so I will continue rambling.

      30mm effective face detection area from an original 40mm, it means 75% effective area, 25% waist, divided per 2 sides = 12,5% per horizontal. The same as for the vertical, so we arrive to "a bit more than 10% from each border" - For a math ass like me this sounds easier than "1/8". (Indeed 1 cake divided by 8 portions will give each folk a 0.125 (12.5%)).

      "a bit more than 10% from each border". This sounds to my ears more diggestible

      Or, in other somewhat ass-twisted formulation, "somewhat more than 1/10th from each border". This is even easier to me than a bit more than 10% from each border

      Kindly notice that "somewhat more than 1/10th" doesn't stick in my mind as 1/8 but the opposite, 1/8 sounds me like less than 1/10th, although mathematically it is not.

      Curiously, in my test there was a books wall (full of shelfs) 1m behind the subject. But within the distance I performed my "kitchinette test", 2.7m from the suject, the face not in the center, the camera remained effective face priority.

      All this was to give myself a round idea of the "face detection" parameters, with my new 14mm lens for street photography. It seems to be quite good. Now let's see the lens in practice. Pictures next month.

      tv/telephone/old toaster. And a computer !


      • #4
        Re: AF Under Stress

        I think you are tackling this far too seriously, with the 14mm just set aperture to F4 (or there about), smallest focus square (though with the 14mm this doesn't really matter) and everything from a few feet to infinity should be in focus.

        With longer FL lenses such as 20mm the focus square size becomes more important, with CDAF the camera will look for best contrast ANYWHERE within the AF frame and will adjust focus from infinity back towards the camera, so will tend to focus behind a subject if the subject does not fill the AF frame, hence the importance of the smallest AF square.

        A trick with the 14mm is to focus on something about 6 feet (2 metres) away and switch to MF, aperture set to F4 or above and shoot away (just don't touch the focus ring)