How do you focus at night?
Here in Alaska, with our long winter nights, we have lots of opportunity to practice night photography. Accurate focusing is especially important and easy to goof-up on a dark, cold night. In addition, most likely you will be using the maximum aperture of your lens, which means that the depth-of-field will be minimum making it essential to have an absolutely precise focus point.
After mounting the camera on a solid tripod, the first step is to set your camera to manual focus. You don’t want the camera attempting to re-focus as you push the shutter button. Some lenses have a mark for infinity focus and you may be tempted to set the focus on this mark. Depending on your lens and ambient temperature, this might work but my experience is not to trust this mark. It is usually a little off – just enough to give you a soft image. If you are using a prime lens (non-zoom) that has an infinity mark you may want to test the lens to see if using mark will be consistent at all temperatures. If so, you are in luck. Focusing at infinity will be easier for you.
Usually the best approach is to find and object (or create a light) at an appropriate distance that is bright enough to manually focus on (moon, street light, etc.). Depending on your camera, this means focusing through the viewfinder manually or using live view and enlarging the image to 5x or 10x to get a precise focus. I like using live view in my DSLRs and live view with focus-peaking in my Micro 4/3 camera and have had good luck using both these systems.
Keep in mind, that once you have set the focus point, you must not touch or bump that focus ring while you are fumbling around in the dark with your camera and tripod to fine tune your composition. (Which is better for fine tuning your setup, cold hands or gloved hands? Cold hands!). In addition, if you are using a zoom lens and you change the zoom setting intentionally or accidently, most likely this will also change the focus point slightly so you should go through the focusing routine again at the final zoom setting before making the image.
Some very experienced night photographers suggest using a piece of tape on the lens to hold the focus ring (and/or the zoom ring) at the precise point. I have not tried this since I worry that the process of putting tape on the lens may move the focus ring slightly. Cold weather would seem to complicate this process as well. But this system works for some, so you may want to try this. My method is to be very careful not to bump the camera after I set the focus.
I hope this helps you in getting sharply focused night shots. Have fun at night!