Hi Dario, if you are having problems with lenses at infinity, especially wide open, it could be several factors:
1) depth of focus is smallest at infinity and wide open, compared to wide open and close focus. Thus every slight misalignment of the whole system can lower the resolution severely. Depth of field is greatest but depth of focus is indirectly proportional to it. Issues like AF inaccuracy, AF sensor/focusing screen/CCD/film/mirror alignment,... 2) aerial haze, termal effects on air (worst with very long lenses) Good light! fra