The Canon 100mm macro stays the same length when you focus; all the parts which move are internal. The Sigma 105mm macro extends by about 2"/50mm when you focus. Having a lens that doesn't extend is more convenient. But the Canon costs more. Only you know whether it's worth the extra money for you.Now someone keep telling me to get the Canon Macro 100mm they said it faster and everthing is internal barrel or something. And that the sigma the barrel extended