28-105mm lens problem

Discussion in 'Digital SLR' started by Pete D, Dec 1, 2005.

  1. Pete D

    Pete D Guest

    I have a 28-105mm zoom that is sharp across the range above F5.6 when auto
    focusing. At max apperture of F2.8-4.0 autofocusing it focuses forward of
    the focus point, if I manual focus it is much sharper. The 105mm end is
    better wide open than the 28mm end.

    What can I do about this?
     
    Pete D, Dec 1, 2005
    #1
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  2. Pete D

    Steve Wolfe Guest

    I have a 28-105mm zoom that is sharp across the range above F5.6 when auto
    It sounds like the AF isn't hitting the spot you really intend on hitting.
    Download one of the various test charts, and find out just where it's
    focusing.

    steve
     
    Steve Wolfe, Dec 1, 2005
    #2
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  3. Pete D

    Pete D Guest


    Yes I have done that, that is how it is focusing in front wide open but is
    fine at everything above F4.0. I would have expected that if it was out it
    would be across the range.
     
    Pete D, Dec 1, 2005
    #3
  4. Pete D

    Pete D Guest

    Yes I have done that, that is how I know it is focusing in front wide open
    but is
    fine at everything above F4.0. I would have expected that if it was out it
    would be across the range.
     
    Pete D, Dec 1, 2005
    #4
  5. Pete D

    Stacey Guest


    Return it for another or for repairs? You may have to send the camera AND
    the lens in to get this resolved.
     
    Stacey, Dec 2, 2005
    #5
  6. Pete D

    Pete D Guest

    6 other AF lenses are perfect so the problem is not with the camera.
     
    Pete D, Dec 2, 2005
    #6
  7. Pete,
    This is a problem on some canon lenses. The question is: is it a
    lens or a camera issue? I have a link somewhere to a canon document
    that acknowledges the problem (a pdf). If you have the same issue
    on multiple bodies, it is due to a lens problem. You can apparently
    send the lens (or camera) to canon and they can recalibrate it.
    Not sure what it will cost. Try contacting canon service.

    Roger
     
    Roger N. Clark (change username to rnclark), Dec 2, 2005
    #7
  8. Pete D

    Pete D Guest

    Thanks Roger, not a Canon and I am thinking that this lens and all others
    like it are just crap, it was cheap, too cheap really and as I said 6 other
    lenses on my Ds are perfectly fine.
     
    Pete D, Dec 2, 2005
    #8
  9. Pete D

    Monty X Guest

    There might be a firmware update for the camera out, check the manufacturers
    website and see.

    Might need warranty repair.

    MontyX
     
    Monty X, Dec 2, 2005
    #9
  10. Pete D

    bmoag Guest

    Its the lens, not your camera.
    The internal gearing mechanisms of many autofocus lenses is not what it
    should be, but if the lens will lock on focus at all the mechanism is not
    likely the problem.
    Autofocus is really a search for maximum contrast, which is why autofocus
    works best at high contrast edges,particularly at low light levels (neurons
    in the visual cortex fire at edge boundaries also, but that is another
    subject).
    A possible explanation is that by some fluke that in the aperture range you
    describe that flare or some other optical effect is such that maximal
    contrast does not coincide with accurate focus under those particular
    conditions.
    If you want another demonstration of the problems that dubious optical
    design can wreak on an autofocus lens go to a camera store and check out the
    Nikon 55-200. I have never experienced an autofocus lens at any price that
    has to hunt and peck beyond and below the focus point for so long: it is not
    an exaggeration to say that you can time the focus with your wristwatch.
    However if you look at a light source through the lens you see a general
    smear which suggests why the camera autofocus sensor cannot find what it is
    looking for.
     
    bmoag, Dec 3, 2005
    #10
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