Canon 30D focus autofocus accuracy with f/1.8 lens

Discussion in 'Canon' started by Doug McDonald, Aug 30, 2006.

  1. I got the 50 mm f/1.8 AF lens for my new Canaon 30D.

    It is much sharper than the 18-55 zoom that comes with the
    camera, at f/3 to f/8 or so.

    The autofocus works good enough on three dimensional
    objects. I'm using only the central focus spot.

    But for flat subjects like books or walls, or more
    importantly focusing on a subject all of which is
    effectively at infinity, it is nowhere near good enough.
    Is this to be expected? I have to manually focus, which is
    not exactly easy with the flat matte screen with no focus aids.

    What good is autofocus if it does not work? Why don't they
    have visual focusing aids like in my old Minolta X-700?

    Doug McDonald
     
    Doug McDonald, Aug 30, 2006
    #1
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  2. Because the focusing screen is relatively transparent, manual focus can
    be unreliable, as can depth of field preview.

    Most DSLR AF does not work especially well unless custom calibrated.
    Fixation in London will do this for the 30D - it involves setting up the
    sensor, AF module, mirror and screen so they are all perfectly
    positioned, and reprogramming the CPU of the camera to tell it this. As
    delivered, the camera contains instructions to modify focus points to
    compensate for small errors. These only show up, normally, at infinity
    with short focus or wide aperture lenses used wide open. You would want
    a camera set up like for aerial work, for example.

    Fixation custom-tune most of the UK national press and news agency
    cameras routinely. They are 'good enough' out of the box, but sometimes
    good enough isn't good enough :)

    David
     
    David Kilpatrick, Aug 30, 2006
    #2
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  3. Doug McDonald

    john Guest

    I recently bought the 350D with the 18-55 kit lens. I too am
    disappointed with the auto focus results, far too many shots are fuzzy
    and are simply not in focus. I've been looking at some of the pics
    taken with my old point and shoot Sony...hate to admit it but they
    compare favourably. I'm reluctant to splash out on a lens in case
    matters don't improve.
     
    john, Aug 30, 2006
    #3
  4. Doug McDonald

    G.T. Guest

    When are your going to give up your life as a troll?

    Greg
     
    G.T., Aug 31, 2006
    #4
  5. It may well be a bit trollish - not owning a 30D I have no idea, but
    I'd be surprised if someone can't easily focus a 1.8 on a decent matte
    screen... I used to *hate* focussing aids..

    (Just for the record it is worth noting that this *isn't* the Douglas
    MAcDonald with the mutiple personality complex, paranoia and ridiculous
    enlarging claims, in case that is what you meant.. If it isn't what
    you meant, then just ignore me... (O;)
     
    mark.thomas.7, Aug 31, 2006
    #5
  6. And I really missed the split image that I had on my OM-1...
     
    Jørn Dahl-Stamnes, Aug 31, 2006
    #6
  7. Doug McDonald

    G.T. Guest

    Interesting. Not only is his name disturbingly similar but he has the
    same writing style. I thought it was an ever bigger troll since the
    last we heard from the Douglas he was incessantly bashing Canon.

    Greg
     
    G.T., Aug 31, 2006
    #7
  8. This was not a troll! It was a question ... a real genuine
    question, "Is this to be expected?" IS IT?

    Doug McDonald
     
    Doug McDonald, Aug 31, 2006
    #8
  9. I can focus the thing just fine on the matte screen ... but
    it takes time, care, and thought. And it probably would not
    be good enough if the thing were diffraction limited at
    f/2.8. With my old focus-aid Minolta it was trivial ...
    just focus until the "shimmer" disappeared.

    Doug McDonald
     
    Doug McDonald, Aug 31, 2006
    #9
  10. Doug McDonald

    AaronW Guest

    Try enabling all the AF points.

    http://digitcamera.tripod.com/#slr
     
    AaronW, Aug 31, 2006
    #10

  11. Please explain why. The manual says that only the center one
    works properly with f/2.8 lenses.

    In any case, of course, if you have s somewhat
    three-dimensional subject, you need to use a single-point
    focus if the desired plane of focus is not at the nearest
    point.

    Doug McDonald
     
    Doug McDonald, Sep 1, 2006
    #11
  12. Doug McDonald

    AaronW Guest

    Not that only the center can work properly, but the center can work
    better.
    If you need that. BTW, I suspect the camera does not use the nearest
    point everytime, but it tries to be more clever in picking a focus
    point.

    But anyway, you were talking about flat scene. With all the AF points
    enabled, maybe the camera can ignore a few points that have AF error,
    and uses the majority that have good focus at a same distance.

    http://digitcamera.tripod.com/#slr
     
    AaronW, Sep 1, 2006
    #12
  13. Doug McDonald

    AaronW Guest

    Canon 50/1.8
    Canon 85/1.8

    Canon 28-135/3.5-5.6 IS
    Canon 70-300/4-5.6 IS

    http://digitcamera.tripod.com/#slr
     
    AaronW, Sep 1, 2006
    #13
  14. Doug McDonald

    derek_c Guest


    What's important for AF is contrast.

    So if you want to focus on something flat, focus on the edge.

    If there's effectively no edge, like a plain flat wall with no colour
    changes, well that would have been a very boring picture anyway!
     
    derek_c, Sep 2, 2006
    #14
  15. By flat I meant physically ... not contrast-wise "flat". I'm
    talking things like books, posters, or scenics all of which
    are at least a couple of hundred lens focal lengths away.
    They have plenty of contrast.

    I've made more tests and the results are bizarre. If I hand
    focus correctly, then carefully switch to autofocus mode and
    do an autofocus, the camera gets the correct result.
    Otherwise, it is usually off.

    This is not, for me, a killer fault, since for such scenes
    I'm a nervous settings fiddler anyway, and will of course
    use the camera's closest zoom view to check things anyway.
    I am delighted how well the camera does for most three
    dimensional scenes.

    Doug McDonald
     
    Doug McDonald, Sep 2, 2006
    #15
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