I have a question about two lenses that are virtually identical. There is the Canon 100mm f/2.8 versus the 100mm f/2.0. I am looking to purchase these for portrait shooting mainly. My question is this concerns prices. On several websites ... the 100mm f/2.8 is $469.95 and ... the 100mm f/2.0 is $389.95. It was my general understanding that the faster the lens at the same focal length, the more expensive. These prices contradict that. The only difference in specs is that the 2.8 has groups/elements of 8/12 and the 2.0 has groups/elements of 6/8. Is this what is causing the faster lens to be cheaper? Thanks in advance!