The Shutter: "The When" and "The How Long"
Just like shutters on a house open and close to either let light in or keep light out, the shutter on the camera controls when, and for how long, light is let through to reach the camera sensor. The photographer determines “the when” by pressing the shutter button causing the shutter to open. Shutter speed, set by the camera or the photographer, determines “the how long.” Slower shutter speeds mean the shutter stays open longer which allows the light from the scene to hit the sensor for a longer period of time. Faster shutter speeds reflect opening and closing the shutter more quickly meaning light from the scene hits the sensor for shorter periods of time.
Why do we care about shutter speed at all? Blurry puctures and really cool effects.
How long can you hold an object COMPLETELY still? Likely not as long as you think. When the shutter is open and collecting light, even tiny movements cause where the scene is hitting the sensor to shift which will cause blurring.
Imagine taking a picture of Bubble Rock in Acadia National Park (shown on right). That boulder is not going anywhere. Mouse over the image to see what happens though if you happen to wiggle the camera while the shutter is open. Camera shake moves where the scene is hitting the sensor while the sensor is building the image. The result is the stationary rock (and the rest of the scene) gets recorded on different parts of the sensor in the single exposure and causes blur.
Like any skill, ability to hold the camera steady improves with practice. Generally speaking, if you need to use a shutter speed slower than 1/125 or 1/60 of a second, you should use a tripod, or other stable platform, to eliminate camera shake. If you are shooting with a telephoto or zoom lens, a good rule of thumb for minimum shutter speed while handholding is 1/focal length of the lens. For example, if you are using a 400 mm telephoto lens, 1/400th of a second would be the slowest shutter speed you would want to attempt without using a tripod.
Even if using a tripod or placing the camera on another stable surface, the photographer still must understand the relationship between shutter speed and motion blur. A photographer could hold the camera as still as a statue and still end up with motion blur.
How? Well, even with a perfectly still camera during the exposure, if a subject is moving fast enough to appear in different places on the sensor while the shutter is open, you get blur. Athletes or water splashes require fairly fast shutter speeds to freeze the action.
If you are a math nerd like the author, take a look at the analysis of the F16 picture to the left. That level of understanding is not needed to do great photography. If you like science and math in addition to the visual arts though, you will find that photography is very mathematical in nature – allowing you to explore the physics, algebra, and geometry involved to the extent that you are interested.
For those not numerically inclined or interested, the value of understanding shutter speed involves the artistic effects that can be achieved by using different settings.
The photographer captured this F16 traveling 350 knots by using a shutter speed of 1/1000th of a second. 350 knots is about 400 miles per hour or approximately 589 feet per second. At that speed, the jet would move .589 feet in 1/1000th of a second (589 feet per second/1000) or about half a foot. The jet was far enough away that 6 inches of movement did not move it very far on the sensor while the shutter was open. The shutter speed was high enough, therefore, to freeze the action and get a relatively sharp picture of the fighter jet.