It looks to me that the only difference between the Sigma 70mm f/2.8 EX DG Macro Autofocus Lens and the Sigma 105mm f/2.8 EX DG Macro Autofocus Lens is the focal length. Technology in the lens appears to be the same, but for some reason they are the same price. Is there a reason the 105mm isnt higher priced than the 70mm?