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
