Focus flange distance....

Discussion in 'Digital SLR' started by LCD, Oct 5, 2005.

  1. LCD

    LCD Guest

    I have a Sigma 100-300 f4 EX. When I first got it, it focused fine. Now
    however it has gone soft on me. At f4 it will focus fine at 135 and 200mm
    producing a very sharp image but not so at 100, 250 and 300 with 100 being
    very blurry. Stopping down improves the situation a bit. I am using it on a
    20D and a D60. It behaves virtually the same on both. I contacted Sigma and
    was advised to have my cameras' 'focus flange distance' (FFD) checked.

    Am I right in thinking that if this FFD were wrong, then it would not focus
    at any focal length? Also that it is highly unlikely both cameras would have
    FFD errors, suddenly.
    LCD, Oct 5, 2005
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  2. LCD

    Tony Polson Guest

    You're right. The problem is almost certainly with the lens.

    You should first test the lens using manual focusing. This is a quick
    and easy test. If you can focus manually at all focal lengths, there
    could be a problem with the electronic interface that is causing the
    camera's processor to think the lens has been correctly focused when
    it has not.

    With autofocus, you should use single shot AF for your tests. There
    are known problems with Sigma lenses and predictive autofocus systems
    used by several camera manufacturers including Canon. Depending on
    the subjects you shoot, this may mean that you should avoid using
    continuous AF mode and stick to single shot AF where possible.

    If the problem is still there in manual and single shot AF modes, I
    suggest that the cams that move the lens groups when zooming have
    probably become worn - or possibly they may have been damaged. Has
    the lens had extensive use? Have you ever dropped it or subjected it
    to mechanical or thermal shock?

    Whether as a result of wear or damage, this can result in the lens
    groups being out of position for some focal lengths. It is perfectly
    possible that they are still in the correct position for other focal
    lengths, which could explain why the lens can focus perfectly at some
    focal lengths but not at others.

    I would recommend that you should ask a competent camera repair
    technician to look at the lens to diagnose the problem. It should not
    cost a lot of money to identify what is wrong. Possibly a CLA (clean,
    lubricate, adjust) is all that is needed, but replacement of the cams
    could be quite expensive. It might be cheaper to replace the lens, so
    ask only for an estimate at this stage.

    At the end of the day, this is a lot of lens for a very low price.
    Externally, it appears well made. It gets good ratings for its
    optics. It has the HSM feature. However, to keep costs down, money
    has to be saved somewhere.

    With Canon L glass, you can be sure that the optical and build quality
    is of a high standard. Sigma lenses cost much less money to buy and
    even less to make, so something has to give. With Sigma, the saving
    is usually made in the internal build quality, which often leaves a
    lot to be desired, but is invisible to the buyer.

    Good luck. Let us know how what happens.
    Tony Polson, Oct 5, 2005
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  3. It is highly unlikely that the camera's focus flange distance could
    change. Either the bayonet flange would have to move, or the sensor
    would have to move.

    It is theoretically possible. As was discussed recently in another
    thread, the depth of focus (the range of film/sensor positions over
    which the image will be sharp) is a small fraction of a mm*. However, if
    this did happen, it would show up on all lenses used with that body.

    Theoretically possible, though extremely unlikely, for one body. The
    odds against it happening simultaneously to two bodies are probably
    something like the odds of you winning the lottery two weeks in a row.

    It is IMO *far* more likely that some element in the lens has become
    displaced, or one of the cams which move the various groups around
    within the lens has become flaky. From my limited experience of Sigma
    lenses (1 out of 3 has crapped out with a faulty diaphragm) and from a
    much greater reported base, this would come as no great surprise. It is
    likely that Sigma are just giving you the run-around.

    The improvement on stopping down is not much diagnostic use, as both
    problems would show this. The fact that it varies with focal length is
    however strongly indicative that it is the lens which is at fault and
    not the body.

    If you can show that (a) other lenses work fine on the bodies, even wide
    open, or (b) that the lens shows the same fault on a third body, you
    should have them (as the UK saying is) "banged to rights".

    *For a subject at "infinity" (i.e. for this purpose a distance very long
    compared with lens focal length) then depth of focus t is given
    approximately by t = 2CN, where C is the acceptable circle of confusion,
    say 0.02mm for a 35mm film or DSLR camera, and N is the f-number. Thus
    at f/4 this would be 2x0.02x4 or 0.16mm.

    David Littlewood, Oct 5, 2005
  4. LCD

    Skip M Guest

    Not only that, but the problem would manifest itself with any and all lenses
    you have, too. If this is not the case, it is undoubtedly the lens that is
    the culprit.
    Skip M, Oct 5, 2005
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