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For a while my Sigma 18-50 f/2.8-4.5 lens has been shaking the viewfinder image once in a while. After asking a related question I settled for the explanation that it was the image stabilisation kicking in. However, now I noticed that the shaking happens only when I zoom in/out around the 24mm focal length. The image suddenly jolts when the lens passes the 24mm.

Looking at the lens from the front I see that the actual lens element jolts when I pass the 24mm mark. Turning the camera off and zooming in/out does not yield the same behaviour. Taking the lens off the camera and zooming in/out also works fine. The AF/MF and IS switches have not effect on the behaviour. Currently I can perfectly work around the issue, but if this is a symptom of some kind it would be good to know.

My question is: what am I seeing and might this be the beginning of the end of my lens?

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It's hard to say for sure, but if the issue only appears when the lens is mounted on the camera and when it's powered on, it does suggest that the issue is something to do with the electrics in the lens. Have you confirmed that the issue is still occurring with the lens on another camera body? If so, does the speed at which you zoom make any difference - e.g. if you zoom in/out very, very slowly, does the jolt still happen? What about zooming with the camera held in different positions to see if gravity plays a role? Anyhow, if the problem is still present then it suggests an electrical fault in the lens, and my bet is on a ribbon cable with a crack in it.

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