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Thread: Jerky Focus for Zoom Lens??

  1. #1

    Default Jerky Focus for Zoom Lens??

    I exercised my 28 - 135mm by zooming out from 135mm to 28mm and immediately focus on something nearby. understand that the lens motor will work and slight jerk will happen while focus is being acquired. I noticed that other then the motor going to work, the image through my viewfinder displaces upon the completion of the AF. Is this normal for a Zoom Lens? Is this behaviour what people call the "counter torque"? Pls help me out here. Thanks a million!

  2. #2

    Default Re: Jerky Focus for Zoom Lens??

    Doesn't sound so much like 'counter-torque' (what IS that?), but more like the lens very quickly hunting and confirming focus, kinda like going back and forth with a very short and quick throw, unless you have a very poorly made or defective lens. Have you tried other samples of the same exact lens on your body under similar lighting conditions?

  3. #3

    Default Re: Jerky Focus for Zoom Lens??

    Thanks for the quick reply. Yes, my lens did what you mentioned. What bothers me is the slight jerk and image displacement the lens gave me on the viewfinder straight after its Focus confirmation. Kind of "not right" Not really sure if this observation is in fact normal for al zoom lenses due to their construction. But I don't see that on a 24 - 70mm that my friend has. Maybe should consider your advise to test out the same lens. Any shops to recommend that will not drive me out just to test on their lenses and walk out straight after?

  4. #4
    Senior Member sammy888's Avatar
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    Default Re: Jerky Focus for Zoom Lens??

    Quote Originally Posted by jk30D View Post
    I exercised my 28 - 135mm by zooming out from 135mm to 28mm and immediately focus on something nearby. understand that the lens motor will work and slight jerk will happen while focus is being acquired. I noticed that other then the motor going to work, the image through my viewfinder displaces upon the completion of the AF. Is this normal for a Zoom Lens? Is this behaviour what people call the "counter torque"? Pls help me out here. Thanks a million!

    What brand of lens you using. Looking at your nick I guess Canon?

    I am not sure about Canon since I am a Nikon user. But are you using a modern day lens that has a built in motor to drive the AF or an older one without a builtin one? For Nikon, the model day AFS for example has a silent wave motor to drive the AF very smoothly and quickly.

    But the older D Nikon Lens especially does not have a motor. It relys on the motor that is in the DSLR body to power a gear system on the lens to move the lens' elements around when it focuses. So there is abit of "torque" going on as that tiny flat screw drive-like bit on the camera body that connect to the len's female identation at back end of the lens.As it need quite a bit of force to crank it back and forth, some "tugging" motion can be felt. I get that and it can be quite loud with my old Nikkor D lens when it is attempting to focus and have a hard time catching the focus..well any AF lens before the AFS would do that. In fact lens that is not AFS but are considered modern like the 'G" also have that. I know I hear it and felt it with my 70-300mm lens. That could be what you are feeling with your Canon.
    Last edited by sammy888; 8th December 2007 at 12:20 PM.

  5. #5

    Default Re: Jerky Focus for Zoom Lens??

    Quote Originally Posted by jk30D View Post
    I exercised my 28 - 135mm by zooming out from 135mm to 28mm and immediately focus on something nearby. understand that the lens motor will work and slight jerk will happen while focus is being acquired. I noticed that other then the motor going to work, the image through my viewfinder displaces upon the completion of the AF. Is this normal for a Zoom Lens? Is this behaviour what people call the "counter torque"? Pls help me out here. Thanks a million!
    Hey, I've encountered this before on my Sigma 18-200mm OS. I went to the Sigma office to check, they said it was normal.

    They even let me test it with another lens of the same model, brand new one and it had the same problem.

    However, I was told that Canon lenses don't have the same problem. Haven't used a canon one before, so I wouldn't know!

  6. #6

    Default Re: Jerky Focus for Zoom Lens??

    Hi Guys, Thanks for the comments. I'm a Canon user and the lense I'm having here is a 28 - 135mm IS USM Zoom Lense. For me, the overall noise created by the stabiliser and ultrasonic motor does not bother me much. Its the annoying jerk upon Auto Focus confirmation that is making me nuts. Its usually the case, once you notice a slight abnormaility, you will forever be focus on the problem...... no matter how minor it is. Think the next best thing to validate this observation will be to test out the same lens from either the service centre or one of the camera store. Thanks both for your time to comment

  7. #7

    Default Re: Jerky Focus for Zoom Lens??

    Hi guys and gals, some updates on the phenominal I see on the jerky focus . This slight image displacement (Jerky) will ONLY happen if I enable the "IS" on my Canon 28 - 135mm. No such issue will happen if I turn "IS" off. For owners with Canon Zoom "IS" Lenses. Is this "jerky" Focus behaviour I'm experiencing normal for Canon "IS" zoom lense or am I having a "defective" Image stabilising Lense that needs servicing? Hope that someone can comment on this observation. Thanks!

  8. #8

    Default Re: Jerky Focus for Zoom Lens??

    Hi moi a nikon user, never tried what you said but current nikon lens with VR does have quite a strong jerk when its VR system kick in

    what i felt was when i mount it my d80 with 70-300VR on a tripod and set it to 2s timer, can see thru the viewfinder that the image slightly jerk and hold stable before the shot was taken, this dont happen on my kit lens which dont have VR, guess the jerking is due to the IR or VR or OS in the lens shifting the lens element around inside the barrel and holding it in place for the final focusing before the shot

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