Originally posted by Na Horuk I have Samyang 14mm, but I would suggest Samyang 16mm. Slightly better aperture, allows filters (not needed for astro, but still good), not as awkwardly wide, but still UWA. But yes, with Samyang the focus is a bit difficult. You can look at the Samyang lens club thread, the last page or two has some astro photos and tips. Focus can be difficult, especially since the focusing rings are often miscalibrated with Samyang lenses. You can calibrate it or just learn where true infinity is on your particular lens. You have to do that before you go out into the field at night, because it will be too difficult out there.
Yes, in regard to total light intake for astro photography the Samyang 16 mm f/2 seems to be the real deal. Looking for an astro-/landscape-lens, I am saving up for it. Ideally I would like something wider (14 or even 10 mm), but it's the only UWA Samyang that takes filters. It also seems to not suffer from the weird moustache distortion that the 14 and 10 mm feature, making it suitable for architectural work.
But I wonder why everyone is having issues with focusing and trying to find infinity on their focusing ring? I simply use live view, find a bright star, then twist the focusing ring to the point where the star's diameter is smallest.