IS stands for Canon's proprietary in-lens Image Stabilizer technology. Usually contained in lens name eg. (EF-S 15-85 mm f/ 3.5-5.6 IS USM) and in big lettering on lens barel (IMAGE STABILIZER). All detail about Image Stabilizer technology can be found here. Other after-market lens manufacturer call their image stabilizer system by different name for Sigma it is called OS and Tamron calls it VC
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