Originally posted by macTak A very interesting lens; Sigma is certainly trying new things. I wonder (I'm afraid I'm still not good enough at comprehending lens designs just by looking at them) if this lens in essence has a built-in tele-compressor (as Olympus was supposed to have done in designing its f2 zooms); that would explain in part how they've managed to get the lens this fast, and it would also explain the rather wide focal lengh. A fun piece of trivia: in 1983, Tokina previwed (but never did release) both a 35-70mm f2-2.5 and 60-120mm f2-2.5 (for full-frame, of course).
Here is the lens construction. In blue the lenses in SLD glass (Special Low Dispersion), in pink the (moulded) aspherical lenses.
Indeed, the two rear groups build a converging system, hence a focal and aperture reducer.