Night photography requires you to understand a few important camera capabilities. Grab your camera and its manual and explore three critical features in manual mode: aperture, shutter speed and the ISO setting.
The aperture is an opening in the lens that controls the amount of light entering the camera, and it works just like the pupil of your eye. Aperture size is indicated by f-stop numbers. A low f-stop number, such as f/2.8, means the aperture is open wide (like your pupils in the dark), and a larger number, like f/22, means the aperture is squeezed to pinpoint size (like your pupils in bright daylight). At night, you generally want to let as much light into the camera as possible, so shooting wide open (at your camera's lowest f-stop) is usually best.
To take a picture, you have to open the shutter. Whether it is physical or virtual, your camera's shutter works a lot like a window shutter. When the shutter is open, your camera's sensor absorbs light to create images. When the shutter is closed, no light enters through the lens. Shutter speed dictates how long the shutter is open and the sensor is exposed to light. To achieve a proper exposure in dark scenes, you'll need to use longer shutter speeds (several seconds, minutes or even hours) so the sensor can soak up enough light to formulate an image.
By tweaking the balance between the aperture and shutter speed settings, you can always find a way to achieve the right exposure for night shots. But there's a third, related factor you have to take into account. Your camera has an ISO (International Standards Organization) setting, which lets you control the sensor's sensitivity to light. Some cameras let you ratchet the ISO from as low as 50 to 6400 or higher.
Increasing the ISO setting is very helpful in low light. For example, if you change the ISO from 100 to 200, you're doubling light sensitivity. Practically speaking, this means you can use a shutter speed that's twice as fast to capture the same amount of light. There is a serious trade-off, though: At higher ISO settings, your camera's sensor creates digital noise.
This kind of noise, which you can't hear, refers to variations in brightness in high-ISO images as a result of the electronic circuitry in your camera. It's an unavoidable, undesirable byproduct that makes digital images look grainy, much like high-ISO film created grain in traditional analog cameras.
Lower ISO settings generally produce less grain than exposures shot at very high ISO settings. Nighttime shooting often heightens the effects of noise because the darkness of low-light pictures makes it easier to see. When you edit your pictures, you'll likely want to use noise-reduction software, which helps minimize graininess and makes your images look sharper and clearer.