I recently bought the Sigma 8mm 3.5 and am confused about focusing for the nadir. I shoot 4 shots at 20 degrees with the focus set on manual between 1 and infinity. That all works out ok but I need a well focused nadir and with my tests at f5.6 and 52 inches distance the best focus is 0.5. Do those of you  experienced with the spherical use a different focus setting for the Nadir???
 
Thanks for your time.
 
Rich Gilman