Manual focus of non-full-time manual focus lenses?

Discussion in 'Digital SLR' started by Wilba, Jun 9, 2008.

  1. Wilba

    Wilba Guest

    I keep seeing warnings that you shouldn't manually focus the Canon lenses
    that aren't Full-time Manual Focus, if they are in AF mode.

    For example, in DPReview's test of the EF-S 18-55mm IS
    (http://www.dpreview.com/lensreviews/canon_18-55_3p5-5p6_is_c16/page2.asp)
    they say, "The manual focusing ring rotates on autofocus, and care must be
    taken not to move it accidentally with the lens set to AF, to avoid damaging
    the motor."

    I can imagine some damage might be done if you stop the ring from rotating
    while the motor is trying to do that, but I find it hard to believe that
    Canon would build a lens to sell in the hundreds of thousands, that is likey
    to break in the first minutes of use from just putting on the lens cap.

    Do you know of any evidence of this kind of damage?
     
    Wilba, Jun 9, 2008
    #1
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  2. Wilba

    Colin_D Guest

    What they mean is, don't focus manually with AF engaged. Turning the
    focus ring will damage the gearing between the ring and the motor. Just
    putting the lens cap on is not a problem.

    This kind of motor is only fitted to inexpensive lenses. The ultrasonic
    ring motors fitted to most lenses, known as FTM lenses, allow touch-up
    focusing after the AF has found focus, e.g. if you AF on a person, you
    can touch up the focus on the eyes without disengaging the AF. If you
    move the focus too far, however, the AF will kick in again, so the
    touch-up range has to be within the hysteresis band so AF doesn't
    reactivate.

    Colin D.
     
    Colin_D, Jun 9, 2008
    #2
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  3. Wilba

    Wilba Guest

    Yeah, when DPReview says "set to AF", I believe they mean auto-focussed, not
    just with the switch set to AF.
    Yes, that's the damage I'm interested in - is there evidence?
    But a bump on the front of the lens while it's auto-focussed would be. I
    find it hard to believe that lenses would be built with such a serious
    weakness.
     
    Wilba, Jun 9, 2008
    #3
  4. Wilba

    ASAAR Guest

    These inexpensive lenses are probably able to withstand a
    reasonable amount of turning force as well as moderate bumps. If
    they would be damaged by the slightest of bumps, a huge number of
    these lenses would be damaged after all these years on the market,
    and if they had been, I find it hard to believe that such a design
    flaw wouldn't have been widely known by now.

    When DPReview clumsily said :
    I'm sure that they didn't mean that if you accidentally touch, or
    accidentally try to rotate the ring the lens will immediately be
    damaged. I'm assuming that if that attempt is made to rotate the
    ring, you'll notice that there will be resistance, and that damage
    won't occur unless you don't take the hint and try to use much more
    than the normal amount of force. If you do that, you'll probably
    end up damaging the lens's internal gears.
     
    ASAAR, Jun 9, 2008
    #4
  5. Wilba

    Wilba Guest

    Exactly. Many people say they are easy to damage, so I'm interested to hear
    about those events.
    OK, so a bump on the end of the lens sufficient to make it rotate could
    damage it in that way. Do you know of any instances of that?
     
    Wilba, Jun 9, 2008
    #5
  6. Wilba

    ASAAR Guest

    No instances yet, but if you'll be kind enough to send me one of
    those lenses, I'm sure that I'll soon find one. Now where'd I put
    my mallet? :)
     
    ASAAR, Jun 9, 2008
    #6
  7. Wilba

    OldBoy Guest


    Believe it or not, most consumer lenses are build that way.
    Lenses with ring-type USM (Canon) and HSM (Sigma) are the exceptions I know
    of.
     
    OldBoy, Jun 9, 2008
    #7
  8. Wilba

    Wilba Guest

    So where are all the broken lenses?
     
    Wilba, Jun 10, 2008
    #8
  9. Wilba

    Wilba Guest

    Umm, I'll do a bit more research first myself before I do that, thanks. :)

    For instance, I just tried to see if there is any difference between the
    resistance of the focussing ring when a lens is unfocussed in AF mode, and
    when it is actively focussing (AI Servo mode). I can't feel any difference.

    That suggest to me that the only thing that matters is whether a lens is
    switched to AF or not. In that case, putting on the lens cap is potentially
    damaging.
     
    Wilba, Jun 10, 2008
    #9
  10. Wilba

    OldBoy Guest


    Note the text: "I know of" :)

    Look for autofocus or back/front focus problems.
     
    OldBoy, Jun 10, 2008
    #10
  11. Wilba

    Wilba Guest

    Now _that's_ an interesting idea. I'm discussing those kind of focussing
    issues in another forum, and that idea hasn't come up. I'll keep it in mind,
    thanks.
     
    Wilba, Jun 10, 2008
    #11
  12. The AF "kicks in" if you ask it to, and only then. And yes,
    you can move the AF away from the shutter button --- which us a
    very good idea, not only for manual corrections but also for AF
    and recompose[1].

    -Wolfgang

    [1] The latter is usually a bad idea when using flash, but I have
    done it --- deliberately, and after changing the active AF
    field --- to get the desired effect.
    I might add that it was too dark for the off center AF
    fields to focus ...
     
    Wolfgang Weisselberg, Jun 10, 2008
    #12
  13. Gone to young men, every one.
    When will they ever learn?
    When will they ever learn?
     
    Blinky the Shark, Jun 12, 2008
    #13
  14. Wilba

    ASAAR Guest

    :)

    When they learn to RTFM.
     
    ASAAR, Jun 12, 2008
    #14
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