Interesting place...kind of unique to see some many trees spread out and with lights.
So why f/18? DOF? When using UWA lenses generally f/8 to f/11 is plenty. For instance, I shoot a 14/2.8 (Samyang) and the MTF testing on this lens actually puts f/5.6 at it's sharpest aperture, but I still shoot it at f/8 most of the time for landscape. At f/5.6 and 14mm the hyperfocal point on my distance scale is 1m (even though it has a 5m mark and then infinity mark, it focuses way past infinity, just a manufacturing QA thing). This means at 1m and f/5.6 everything 1m away and beyond is in focus. Any way, I finally went to your flickr stream
and added you as a contact. I looked at the "original" size and it just seems really soft across all of the frame. I'm just providing C&C hope you don't mind. The exposure is just about there if not just fine, but it's the sharpness/crispness of the shot that IMHO is lacking.
I think this is due to two factors, which are long exposure and possibly diffraction due to the heavy f-stop. The long exposure most likely the heaviest of the two. Even on a perceived still night, leaves move more than you think.
Go back and try a 5 second exposure at f/4 or better yet, first test your lens focusing ability and understand it's hyperfocal points...I'm sure that someone has tested this FABULOUS lens. Go back and shot at f/2.8, f/5.6 and then f/8 (adjust such frame so that you get proper exposure, maybe about 1EV more than you show here with keeping the shutter speed no slower than say 5 seconds and ISO less than 1600. Then compare each frame and see what you like best.
I bet that lens' sweet spot is around f/6.3, in fact, I'll see if I can pull up the blur index from slrgear.com and post it back here.
