Extend exposures are great for capturing cityscapes, star-trails, moonlit valleys, and lightning. In this case, we are concerned with pictures made at night with exposure times that last from several minutes to many hours. If you set your ISO rating high enough, you can also get good night pictures with only a few seconds of exposure. These may look nice in small format, but digital noise will probably be more obvious if you make large prints. In all of these cases discussed here, a tripod and locking cable release are essential; you won’t be able to keep your finger on the shutter release the whole time without moving the camera.
Beware of digital noise in long exposure photography. Cameras vary widely on the frequency of noise they produce so you should experiment to determine your camera’s capabilities. Always choose the lowest ISO setting on your camera for long exposures to minimize noise. Noise is produced by the duration of time electricity runs through the sensor and the higher ISO settings pick up even more of the noise produced by that electricity. Hence, higher ISO images will have more noise than lower ISO images.
One of the first tricks people like to try with night photography is using exposures so long that the scene looks like a daytime shot. The longer the exposure, the more “daylight” the picture will appear to have. This only works with rural landscapes where the moon is out. The moon reflects sunlight, albeit at a significantly reduced rate, and a full moon will yield a picture that looks like a daytime shot using ISO 100, f/3.5 and an eight minute exposure. Less moonlight requires longer exposures, but it’s not a linear equation. In other words, you can’t just do the math to figure out how much time is necessary, you have to experiment.
Be aware of any external artificial light such as lampposts, streetlights, or even the ambient glow from nearby cities. Even the dimmest light can end up appearing much brighter than you want it to in the final exposure, sometimes to the point of ruining the photo if your exposure time is particularly long. Another long exposure trick that is often used for photographing buildings in near darkness is to make the exposure so long that cars and people passing by don’t register. This can be done at night or early morning, but will look like it was photographed in broad daylight with no one around.