Re: Whay does my Canon 30D's autofocus work so badly on many things?

Discussion in 'Digital Photography' started by Paul Furman, Apr 12, 2008.

  1. Paul Furman

    Paul Furman Guest

    It's a similar technology to the split prisms in an old manual camera.
    I'm not sure why you are having problems but perhaps this explains it
    better:
    http://en.wikipedia.org/wiki/Autofocus#Passive_autofocus
    "Phase detection"

    lid wrote:
    > I'm asking a question.
    >
    > I used to think that my camera or lenses had a focus problem.
    >
    > It's a Canon 30D and various lenses. The problem si teh same with
    > all lenses.
    >
    > It often simply guesses wrong with autofocus.
    >
    > So I tested it. I tested it first with flat text material slanted
    > slightly away from perpendicular to a line from the camera, and
    > found unreliable autofocus. Then I tried with flat subjects
    > carefully placed perpendicular to the line of sight, and it works
    > quite nicely using autofocus ... or manual focus.
    >
    > I should add that the autofocus is set to use only the center
    > focus point. The problem is not dependant on whether I'm
    > using a lens that is faster or slower than f/2.8, which is,
    > I believe the point at which it switches to "fine focus mode".
    >
    > But used on real subjects, it simply is poor at autofocus.
    > Sometimes it will focus in front, sometimes in back.
    >
    > I read here that this camera in fact lies about the size of the
    > focus areas: I had assumed that they were the size of the little
    > squares in the viewfinder, but somebody says that they are
    > three times larger. That could be the problem, of course, with small
    > subjects.
    >
    > How does the autofocus work? I read about "vertical and horizontal lines"
    > but it seems that the center focus point uses both.
    >
    > It is simply fooled by things at different distances within its
    > focusing area? Does it try for an average? The front? The back?
    > OR is it just junk and I'm going to have to manual focus
    > on ANYTHING of importance that is not flat and perpendicular
    > to the line of sight?
    >
    > What do pros do ... other than of course take types of picture
    > where the problem does not arise ... it's not a problem
    > with distant landscapes or portraits (where the person more
    > than fills any possible center focus area): there the autofocus
    > works OK.
    >
    > But say I'm taking picture of geese waddling around in the park.
    > The goose's head is simply not going to fill the center focus
    > spot.
    >
    > Oh yes ... please do not say "you should have bought Nikon".
    >
    > I'm very happy with the camera otherwise: a lot of the stuff I take
    > is perfectly obviously not possible for autofocus at all, such
    > as picture of flowers or bugs, where I have to decide "artistically"
    > where to focus. But the damn thing should focus on a goose!
    >
    > Doug McDonald
    Paul Furman, Apr 12, 2008
    #1
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  2. Lawrence Glickman wrote:
    > On Sat, 12 Apr 2008 15:20:50 GMT, Paul Furman <>
    > wrote:
    >
    >> It's a similar technology to the split prisms in an old manual
    >> camera. I'm not sure why you are having problems but perhaps this
    >> explains it better:
    >> http://en.wikipedia.org/wiki/Autofocus#Passive_autofocus
    >> "Phase detection"

    >
    > Or maybe this
    > http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/5700/AUTO-FOCUS/Auto-Focus.html
    > "Contrast detection"


    The DSLR (such as the 30D of the subject) uses phase-detection rather than
    contrast-detection (except sometimes when a live-view function is being
    used, which the 30D doesn't have).

    David
    David J Taylor, Apr 12, 2008
    #2
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