For some long exposure shots, "slow" might not properly describe how long you keep the shutter open. As we mentioned on the last page, you could want it locked open for hours! Without a specific length of time in mind, locking the shutter open with a shutter release and closing it again a few hours later will get the job done. But that technique isn't always ideal. What if you want to capture a very specific length of time, or want to tightly control the amount of motion blur created by moving objects within the shot?
Easy answer: Let your camera do the work. Some cameras offer a shutter priority mode, which allows you to set a shutter time and have the camera automatically adjust aperture (which affects depth of field) to compensate. Choosing shutter priority and selecting a very slow shutter speed -- like 10 seconds or 30 seconds -- will allow you to capture a great deal of light and photograph some very cool motion blur. It's possible to set a slow shutter speed without using shutter priority mode, but you may have to adjust the aperture manually as well.
Setting your camera's timer for 10 or 30 seconds (and using a tripod, of course) is the perfect way to compose those cool light trail photos associated with slow shutter speed photography.