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I recently bought a Canon EOS Rebel G with a Sigma 28-200mm 1:3.8-5.6 UC and Tamron 1.4 extender for $170 (or around 350 quid). From what I've noticed so far, the body's in superb shape as are lenses, though auto focus seems to be an issue (or non issue as my case may be. I like manual focus for still work). I was wondering if this was a sound investment.
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The autofocus (or lack of) could be a result of using the extender. With the 1.4x extender you tend to lose about a stop of light, this reduction of light tends to cause a bit of trouble with AF. This is why extenders are often used with larger aperture lenses if AF is desired.
In your case, however, since you prefer to MF most of the time, its not an issue. As for whether or not its a sound investment, as long as you get the shots you want, it was a good choice.
It "should" be able to autofocus OK with the TC at the short end of the lens, since that combination is a tad wider than f5.6. It might cope in good light at the long end.
If it doesn't try to AF at all with the TC, then it could be that the TC is "telling" the camera that it's present and the camera is disabling the AF because, for much of the lens zoom range, the max aperture of the combination is smaller than f5.6, which is the AF spec for that camera.
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