Focusing infinity with 12-50mm. Is it possible?

There are not dumb questions at this time!

I'm quite sure it was MF. There was the MF image on the screen.
How can I set the mode you say? (MF+S-AF?)

The fact is that with the first lent (20mm) I did the same operations and I succeed in focusing..

PS: the screen was completely dark; The night was very deep;
When the photo was taken it was correctky exposed.. but unfocused.