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The EF-S mount sticks a little farther into the body of the camera so the lens won't physically fit on the 5/1D series body. Nothing really "happens", it just won't fit. And I hear good things about the Tamron 17-50, which is very similar to the Sigma 18-50 F/2.8 EX DC that I used to own. The problem with those lenses (Tamron/Sigma) is that I know the Sigma doesn't have full-time manual focus, lacks a USM-type motor, and obviously no IS. I think the Tamron has basically the same features. While both are F/2.8, which is nice, they still lack the options that I was willing to pay for when I went with the 17-55 F/2.8 IS. As a previous owner of the Sigma 18-50 F/2.8, I really appreciate the jump in quality when I went with the Canon. If you have the money, the 17-55 F/2.8 is simply the absolute best all-around lens for a 1.6 crop body, hands down.
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