Anyone who works with photography knows that light is the key element to any good photo. In fact the word photograph itself is comprised of the two Greek words photos and graphe meaning light (photos) drawing (graphe). That said, it may seem a bit odd to talk about taking photos at night.
Night photography has always been one of my favorite subjects (see here) and within the last year i have began to play with astrophotography which is a bit more challenging because it has to be done basically in the darkest place you can find... basically going against everything you normally do in photography.
Camera autofocus systems use contrast, or the insections of dark and light, to find focus. So what happens when you are in the middle of nowhere and all you have is a sea of black around you? You have to manually focus. During the day manually focusing isn't a huge ordeal, but at night it can be very difficult. It often requires finding a single bright object in the distance such as a star or a light and adjusting the focus until that object is sharp. Sometimes i have to use a flashlight or a laser pointer to illuminate an object while i micro-adjust until peak focus is found. Finding a pre-illuminated object is easier needless to say.
When it comes to exposing the photos you get a whole new set of problems. This may sound obvious, but stars move across the sky (we move, not them) throughout the night. And while the naked eye can't see this movement, if you take an exposure longer than about 20 seconds you will begin to see that movement show up in your photo in the form of the star becomming stretched or linear instead of round. Getting your exposure down to 20 seconds requires a fast lens or bumping your ISO (sensor sensitivity) on your camera with the latter introducing more digital noise (grain) into your photo. This is fixable in post-processing but can be a pain and can reduce overall photo quality.
There are a number of unique and challenging issues with taking photos at night, but the end product can be very rewarding and just as unique.