Before you do any shooting at all you're going to need to learn how to focus, now focusing at night is nothing like focusing in the day. Depending on which lens you are shooting on focusing can be done in different ways but you can't just be sat in the pitch black and press auto focus and the image will focus, the camera will struggle to focus and the lens will just keep turning. Auto focus is possible but certain preparations have to be made. Even manual focus is not as simple as look through the lens as focus, again certain things have to be done to make this possible.
Here are ways I have used before, and have picked up these techniques from other people or have learned them myself from trial and error. All of these methods have been successful for me but I always do a test shot before doing my actual long exposure just to check my focus.
The Infinity Method
This is not a method I tend to use a
lot because its accuracy is not brilliant. Look on your lens with a torch and
set you focus in manual focus to infinity. Looks like an 8 on its side. This is
only valid if you are 3 meters or more from your subject. The problem here is,
guessing 3m away from your subject which when shooting wide-angle is hard
because you are close and low to the subject. The other problem is people tend
to go past infinity, which just throws the whole image out of focus. I have had
friends use this technique and come in with a whole nights worth of images out
of focus because of this technique. Only bit of advice I can say is carry a
tape measure to measure you 3m and make sure you don't go past infinity. The image below is how infinity should look on your lens.
The Torch Method
Find the thing you want to focus on, go and place a torch where you would like the focus to be facing towards the camera lens. Go back to your camera; zoom in focus on the light, and go get your torch. This is a good method if you have a stand 18-55mm lens, not so good if you have a wide-angle lens. You can use either auto focus or manual focus here, if the torch if not very strong auto focus may be bad so manual is best. You can also do this technique in reverse and shine a torch or a laser pen works well here, and shine it on the point of focus you would like then use either auto or manual focus, again auto may be bad so manual works better, but only if you have a lens with a good zoom on. The reason this is not successful with wider lenses is because even zoomed in you are very wide and can't see where the torch is very well and can't see weather it is focus or not even on auto focus.
The Flashgun Method (Auto Focus Only)
This is one of the best methods in my opinion and it's the one I use every night. This only works if you are using an infrared flashgun that is the same make as your camera: Canon 5d mkii/Canon Speedlite 580ex. These 2 are compatible with each other and work very well. At night I can mount this to my camera before an exposure, put the camera in to auto focus press the shutter half way down and the infrared beam on the flashgun will fire, hit the subject I want to focus on and focus on it. I then remove the flashgun and put the camera in to manual focus because if you don't the camera will try and refocus when you go to take the shot. This works every time for me. See picture below.
The Live View Method (Manual Focus Only)
This will only work on camera with live view mode, this is where you press a button that shows you what you would see in you view finder on to the back of you LCD. Only problem is that we are still shooting at night so the screen will appear black. So you need a quite powerful torch, and you shine it on where you would like you point of focus, this will then appear on the screen, use your directional button to move the square on to the point of light, then zoom in with the buttons you would usually use to zoom in on an image you are viewing. You can zoom in once to x5 and twice to x10, from this point you can use manual focus to make sure the image is sharp. Once the image is sharp you can press the live view button again to resume normal shooting and you are ready to go, just remember to have your lens to to manual focus. See picture below.
