Now that you've practically mastered ISO and Aperture, we'll turn our attention to shutter speed - the last remaining setting in what some photogs call the "Triangle of Exposure." Your shutter speed is just that – the amount of time that your shutter is open. This is measured in seconds and can also be set manually by you or automatically by the camera, according to how long the shutter needs to be open in order to let in the necessary amount of light for a correct exposure.
Related terms and phrases: long or short exposures, seconds, exposure time
You'll find your shutter speed is adjusted with the small dial found by your shutter button when in Tv or Manual mode. (Some cameras offer a "bulb" mode which keeps the shutter open for customizable amounts of time.) The shutter of your DSLR is similar to your car engine in that it has a life span. Amateur and mid-range cameras have an approximate lifespan of about 50,000 shutter clicks while higher end professional grade DSLR's approximate 100,000 images.
The shutter speed is measured in seconds – most often the shutter is open for a fraction of a second. Only the denominator is displayed in your viewfinder. Long shutter speeds of whole seconds are displayed with second marks (4”). The larger the denominator (numbers again, hang in there), the faster the shutter speed. That means the shutter is open for a very short amount of time (i.e. 1/500 of a second), and therefore ample light is needed in order to make a correct exposure (that's where your aperture training comes in!).
If you're looking to capture a city skyline at night, you'll need to use a longer shutter speed (smaller denominator or even whole seconds). Keep in mind that the longer your shutter is open, the more important it is that your camera keep still. Most people recommend using a tripod when shooting 1/60s or slower. Image stabilization is being added to an increasing amount of lenses, allowing the user to shoot with a slower shutter speed and receive less camera shake. A rule of thumb regarding focal length and shutter speed: it's a good idea to keep the denominator of your s.s. larger than the focal length of your lens. (Shooting with a 200mm lens, keep shutter speed at least 1/250s. Lens with a fixed 50mm focal length should use at least 1/60s s.s.)
As with aperture, shutter speeds double in amount:
1/500s, 1/250s, 1/125s, 1/60s, 1/30s, 1/15s
The smaller the denominator number, the longer the shutter is open and the more light is allowed in. Adjusting your speed from 1/500s to 1/250s will double the amount of light exposing the film/hitting the sensor.
So what can shutter speed do for you? It all depends on what kind of movement you want to capture in your image. You have the power to freeze all movement, or to capture an image full of movement and blur. A faster shutter speed freezes the movement; a slower shutter speed allows the movement to blur.
The images below were taken with a slow shutter speed. The shutter was open for 1/5s during the exposure and I followed the moving subject so that it was clear and the background became blurred. Had I remained still, the car would become the blurred motion.
F/29 at 1/5s
When your subject is stationary, you can also achieve motion blur by using a slow shutter speed as in the photo below.
F/22 at .3s
Because the shutter is open for an extended period of time, the aperture is quite small. The time of the exposure lets in enough light for the aperture to remain narrow. Need to freeze the movement? Then make sure your aperture is wide enough to let in enough light for correct exposure over a very brief amount of time.
These photos were taken last summer during a family vacation. My four younger cousins are water sport masters as evidenced by the mad air they're getting. I felt successful when I remained attached to my board...
Images 1&2 : F/4.5 at 1/2000s Image 3 : F/4.5 at 1/2500s
So to bring it all together – ISO, Aperture and Shutter Speed are three adjustable elements that work together to create a correctly exposed image. Remember that you have the most control over these elements when shooting in Manual mode. A few things to keep in mind:
A change in aperture or shutter speed requires a change in the other. Opening your aperture a bit wider? This will let in more light, allowing the shutter to be open for a shorter amount of time. Increasing one will allow you to decrease the other to make a correct exposure. (For example, if a correct exposure is a result of F/2.2 at 1/800s, a correct exposure will also result from F/1.4 at 1/1600. More light is allowed in with the second combination, so a quicker shutter speed is acceptable.)
Shooting in low light? Keep in mind that you can always increase your ISO, but watch out for that grain. Other low light options include opening your aperture wider to let in more light or increasing the length of time the shutter is open.
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