I have decided to upgrade from a Rebel XTi to a 40D. I am fairly new to photography, therefore, unfamiliar with lens options. I mostly take portrait, close up photography of my children. I like the background blurred out in alot of the pictures. The store I am purchasing from recommended a tamron 17-35 mm lens - he said it would let in more light than my canon 18-55 mm lens and give a crisper picture. He also said it would blurr out the background easily. I will also purchase another telephoto lens in the future - but any recommendations for now. Should I consider a canon IS lens? Thanks