I'm about ready to purchase an M6. I'd like to use just one lens, a 35mm. I know there's f/2 Summicron's both ASPH and pre-ASPH. I'm just going to be shooting black and white, but still want an excellent lens. Is the ASPH really worth the extra cash? Also, is see there is an f/2 Biogon ZM T* made by Carl Zeiss. How does this lens compare to the ASPH and pre-ASPH lenses?