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Old Jun 3, 2007, 2:59 PM   #1
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As you might recall I was asking about zoom lenses a short while ago. Well I have taken the plunge and bought the Sigma 70-300mm The thing is, the new Sigma seems to have the same sort of problem/issue as the old Sigma.

If I point the camera at a scene, e.g. a flower bed at the bottom of the garden with one particular flower head at the centre, with a focal length of 100mm and let it autofocus and then zoom in to 300mm the flower is now slightly out of focus and needs to be refocussed to be sharp. I hope that made sense.

The aperture is a constant f8.

Do I have a problem? If so, with what? I just assumed that the focussing would be the same regardless of the focal length if the camera is pointing at the same subject.


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Old Jun 3, 2007, 3:28 PM   #2
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I doubt that there is anything wrong. Depth of field at 300 is shallower than at 100, so it is possible that the centerpoint of both frames was not the same, Try focussing as 300 first and back off to 100, and it should still be in focus - if not, then you might have a problem. Or try it your way on a flat subject, like a brick wall, and focus should be the same.
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Old Jun 3, 2007, 3:47 PM   #3
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Why not zoom to 300mm first and then let the camera autofocus the lens?
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