Image from: http://www.interfitphotographic.com/Strobies%20Tutorials/images/camera-setting.jpg
Shutter speed is an important element in night photography. It controls the amount of light fall onto the image sensor. Here is a simple formula to memorize the rule for shutter:
Fast shutter=Less light comes in, freezes movement
Slow shutter=more light comes in, blurs movement
For city scene photographs, a shutter speed of 8-15 seconds is usually used. For country side photographs, a shutter speed of slower than 30 sec is recommended, a shutter speed of slower than 30 sec will be used. In order to use a shutter speed slower than 30 sec, Bulb setting will be used. Bulb setting allows you to control the shutter speed manually. To use Bulb setting, you can dial the mode dial on your camera to the letter B, or keep slowing down the shutter speed until you see the word 'Bulb'. Keep holding the shutter button to keep the shutter open. But holding the shutter button will affect the quality of the picture, so Bulb setting is usually used with shutter cord
Aperture is like a gate in the lens, it controls the amount of light going through the lens. Here is an formula to memorize the rule of aperture:
Large aperture (e.g f/2.8)= More light come in, smaller depth of field, which means more area is blurred
Small aperture (e.g F/8)=Less light come in, larger depth of field, which means less area is blurred
During night photography, I suggest to use small aperture like F/8-11, not just because we need enough depth of field, but also better image quality. Most of the lenses have their best optical performance at F8-11. Small aperture extends the exposure time too, and it makes the motion blur effect stronger.
When using zoom lenses, you will notice the aperture number becomes smaller, like the aperture is F3.5 at 18mm, but it become F5.6 at 55mm. This is normal in most lenses. Top model zoom lenses have something called Constant aperture, which means the aperture number will not change even you zoom.
We can identify zoom lenses with constant aperture by looking at it's name:
Canon EF-S 18-135mm f/3.5-5.6 IS, this lens doesn't have constant aperture.
Canon EF 24-70 f/2.8L USM, this lens has constant aperture.
The pattern is simple. If there are two aperture number in the lens' name (something like f3.5-4.5), it doesn't have constant aperture. If there is only one aperture number in the lens' name, it has constant aperture
ISO speed means the Sensitivity of the CMOS/flim. High ISO speed means the CMOS/flim is more sensitive to light. Higher ISO speed will add more noise to the picture. Nosie is something that lowers the image quality, it makes the picture losing it's details and adding ugly dots to the picture. So always use low ISO for night photography.
The bad news is, auto focus don't work well at night. The only way to focus at night is to use manual focus. Most of the DSLR offers a function called liveview. You can use the LCD as the viewfinder, so that you can see things clearer. For taking picture of things far away(such as the sky, landscapes), turn the focus ring to Infinity, which a mark of Horizontal of the number 8, or one end of your focus ring, depends on which lens you are using. Then use liveview function to check the focus. If you want to focus on objects close to you. You have to use your torch to help you Illuminate the subject and turn the focus ring until you get the result you want.