One of my favorite comic strips is Calvin and Hobbes. One particular strip starts with a series of Calvin making atrocious faces. The last panel show Calvin’s frustrated father holding a camera and asking, “Can’t you just hold still for 1/100 of a second?” One hundredth of a second is far faster than the blink of an eye, but in that length of time (and often in even less time) your camera can capture an image that will last you a lifetime. The length of time the camera exposes the sensor to light is measured in fractions of a second. That exposure time is determined by how long the shutter is open.
The shutter is a small gate between your lens and sensor that opens and closes, controlling the length of time light has to reach the sensor. How long the gate is open is called the shutter speed. The shutter speed is probably the most easily understood part of photography and its influence most easily seen in a picture. The shutter speed will determine how bright the image is and whether motion is frozen or blurred. The shutter speed is measured in fractions of a second, which means the second number in the fraction indicates how long the shutter is open. The larger the second number the faster the shutter speed. A shutter speed of 1/150 is faster than a shutter speed of 1/50. A shutter speed of 1/10 is faster than 1/5 but slower than 1/25. As with all three parts of the exposure triangle, there is some given and take with shutter speed. A shutter speed that is too slow can lead to blurry pictures, either from the subjects movement or camera shake from the photographer. A very long shutter speed (seconds long) can also add noise to the picture. A shutter speed that is too fast will not let enough light in to properly expose the subject. Determining the right shutter speed for a given image is as much an art as a science.
Determining the proper shutter speed for an image depends much on what you want to accomplish with the final image. In most situations your shutter speed decisions will be driven by what you are trying to capture and what you are trying to communicate in the light available. A picture of a running back crossing the goal line will dictate a much different shutter speed from a flower macro or a flowing waterfall or a red sunset. When trying to communicate or capture motion, shutter speed is going to be the first decision with aperture and ISO being adjusted for proper exposure. When taking a picture of a waterfall in which the goal is to capture the water flowing in a long white ribbon, the shutter will have to be at least one second in length. That decision is made first, with the ISO and aperture stopped down to provide the right exposure at that shutter speed. If the goal is to capture a crisp picture of a runner, a shutter no slower than 1/250 is probably going to be necessary. If you want to capture a panning shot of a bicyclist with the crowd blurred out, a slightly slower shutter will be necessary to create the blur, but not so slow that the bicyclist begins to lose sharpness. On the other hand, if you are taking a picture of your son or daughter posing in the living room before a big date, your shutter speed will be determined at the same time as your aperture and ISO. The goal would be a clear, bright picture of them, probably with a blurred background. That means your aperture will be wide open, but your shutter will most likely need to be at least 1/60 to avoid any blur from accidental movement. Since the picture is indoors, the ISO may need to be a little higher to keep a fast enough shutter speed. When taking a macro photo, often the aperture will be set very small (larger number) so the shutter speed will be determined to capture the right amount of light at the desired aperture setting. As with many aspects of photography, experimentation and practice are very important here. With the shutter speed there is very rarely one right answer, much depends on what the photographer is trying communicate through the picture.
Here is one example of how to determine the right shutter speed. The picture at left was taken with shutter speed of 4 seconds. I chose that shutter for two reasons, first, the grasshopper was in the shade (helpfully caused by my home made light blocker and noise generator, called a DAVEY) which immediately means the light is at least half as bright as direct sunlight. The other reason was the insanely small aperture I chose (f/57) to ensure as great a depth of field as possible. I found the right shutter by watching the internal light meter in my camera, stepping down the shutter speed until the camera was registering the exposure I wanted. Very simple, quick process that started with my goals and the available light before I moved to the final decision of shutter speed.
Let me share a few quick tips to help give guidance in selecting shutter speeds. First, to keep pictures sharp when shooting without a tripod, use a shutter speed no slower than the focal length of the lens. If you are shooting with the lens zoomed to 100mm, then it is safest to have the shutter speed set at least at 1/100. With the new vibration reduction lens, you may be able to get away with a slower shutter, but this gives a good starting guideline for your minimum shutter speed. Secondly, when intentionally blurring an image to communicate movement you need to be careful to slow the shutter enough to make the blur appear intentional and not accidental. Don’t leave the viewer wondering if the blur was accidental, make it clear that you want that portion blurred out. Waterfalls pictures with the flowing water blurred to a fine white ribbon are very attractive, and this only happens with a shutter speed slow enough to blur the water into one solid string. A slow shutter speed of 1/5 may blur the water, but it will not capture the same flow. A shutter speed set to around one second will blur the water and clearly communicate the flow. Third, when taking pictures that require a slow shutter speed, camera shake will become a big issue. To help prevent inadvertent camera movement, set your camera on a solid platform and use the timer delay function to snap the picture. With the timer delay, you can focus and get everything set, push the shutter release and then stand back with little risk of you accidentally moving the camera during the exposure. Lastly, use a tripod. Many pictures cannot be taken without a stable support for the camera. Tripods are easy to purchase and easy to use, so if you are serious about taking good pictures a tripod is an indispensable tool.
To help put things into perspective with the rest of the exposure triangle, a full stop change in shutter speed occurs when you double or halve the time the shutter is open. A full stop always means the light reaching the sensor has been cut in half or doubled. Slowing the shutter speed from 1/100 to 1/50 is a full stop increase in the light (the light hitting the sensor has been doubled). Increasing the shutter from 1/100 to 1/200 is a full stop decrease in the light (the light reaching the sensor has been cut in half). As with most things in photography, to really get a grasp on shutter speed you need to do two things, practice and experiment. Don’t be afraid of mistakes, instead use them as part of the learning process to take control of those fractions and make much better photos.