i've got a 150 sironar-n, late 80's vintage. i just aquired it and haven't used it yet. i'm sure it will work perfectly well, but what i'd mainly like to know is the difference between this lens and the 150 apo sionar s. i've never used this lens either but have seen and heard much hype over it, which i believe to be true. but what exactly are the differences?

thanks
jeff