I use a shiny (not the supercheap black one but neither the luxury ones that come at silly prices) M42 to EOS adapter from Ebay, it focuses to infinity. I also have ZF lenses with ZF to EOS adapter and they focus to infinity. ZSs have a little lever to open and close the iris with a flick, which can be handy for focussing if you have no focus shift issues. The Planar 1,4/50 has them so the lever is not too useful.

Which adapter are you using on this ZS lens? Does it focus to infinity?