That, my friend, is your camera’s shutter mechanism. Without it I wouldn’t be writing this article about shutters and this website wouldn’t exist.
Behind film or a sensor, the shutter is the most important part of your camera and timing it is the key to good shots.
When the shutter opens to a specified aperture, light begins hitting the sensor and and is recorded until the shutter closes and light stops hitting the sensor. Your camera then processes the recorded light and makes an image for you.
You have two options for selecting a shutter speed: manually and automatically. To have your camera automatically choose a shutter speed for you, just set it to one of it’s preset shooting modes, like sport, snow, or portrait. Your camera then uses a light meeter to determine the correct shutter speed for the shot. Most cameras have shutter speeds that range from 1/4000 of a second to 30 seconds, and BULB mode, where you keep the shutter open as long as you want. Nothing too complicated.
You could also choose your own shutter speed if you want to give your shot a creative effect, such as blurred water or only having only the foreground exposed.
To select your own shutter speed you will either want to shoot in Manual or Shutter Priority if you camera has it. In manual you get to select all exposure settings (shutter, aperture, ISO) for your shot and your camera chooses nothing. In shutter priority mode, all you do is select your shutter speed and your camera picks the aperture and ISO.
To select your own shutter speed you need to take into account the situation you’re shooting in. Take note of the time of day, whether or not your subject is moving, what you want with your subject, where shadows are, and how well lit your location is. Your camera’s light meeter can help you make a good guess on shutter speeds. The nice thing about digital cameras is that if you mis-calculate your shutter speed, you can just delete the photo and take another with the correct exposure.
If your situation is really bright, like on a cloudless day, you will likely be using a fast shutter speed so you don’t have any white wash-out. Fast moving subjects like sporting events or cars also require fast shutter speeds so they don’t become blurry.
Things that require the shutter to stay open include things like blurring moving water and astrophotography.
You should also note how wide your aperture is when selecting a shutter speed. Wide apertures require fast shutter speeds (in most cases) and small apertures need longer shutter speeds.
As with all things, including photography, practice makes perfect!