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11-20-2009, 05:54 PM   #1
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Focus "creep" on 18-50

I (more or less) just got my K-x, mounted the Sigma 18-50 2.8 and, of course, played around with the video mode. What i noticed was that if i manually focus and then zoom in or out, the movement of the zoom ring seems to "take" the focus ring with it to some extent, basically screwing up the shot.

Is this to be expected of lenses below, say, 1000 bucks or do i have a faulty unit on my hand? If the latter, should i just return it to the seller(still have 14 days left for no questions asked return) or is something the lens warranty would cover and i could somehow get fixed?

11-20-2009, 08:48 PM   #2
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Check first if it's a good copy. If it's not return / exchange it. If it is then send it to sigma for the focus ring repair. Although I've heard that Sigma sometimes think that some things we see as a defect aren't actually a defect and will refuse to fix it... YMMV.

FWIW my DA 10-17 is like this as well when going from 10 to 17, and that lens is no slouch when it comes to build quality.... the tolerance is too tight probably. My other AF zooms doesnt suffer this though.
11-20-2009, 09:03 PM   #3
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I'm not sure if the zoom is making the focus ring turn (could very well be a faulty lens) or if the problem is much simpler. The focus at 18mm will be different than the focus at 55mm. You cannot zoom in, focus, and then zoom out and have the area previously in focus stay in focus. This lens doesn't work that way. And very few do.
11-20-2009, 09:22 PM   #4
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Yeah this is normal, focus after zooming

11-20-2009, 09:26 PM   #5
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Yeah, there are two types of zoom lenses - Varifocal and perifocal - It sounds like your lens is varifocal and changes focus when the focal length is altered.
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