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the 28-300 is not a bad lens, although I personally dont like lenses that go from wide angle's through to long focal length, the sigma 70 (or is it 80) to 300 is a better lens..
Internal focus (IF) simply means that the element that is moved in order to focus the lens is inside, as opposed to some of the older (cheapo) lens's where the whole front element would rotate and move in and out in order to focus.
IF makes use of specialized filters (like circular polarisers) possible, and also usually speeds up auto-focus, because the motor drive is only moving a small internal lens cell as opposed to the entire front section of the lens.
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