A shutter is a mechanical device that controls the length of time that the film or sensor is allowed to be exposed to whatever light is passing through the lens. The shutter speed is that length of time. Wait just a minute. How can shutter speed be a length of time when speeds are expressed in units like miles per hour or meters per second, and times are expressed in units like hours and seconds? The answer is that SHUTTER SPEED IS NOT A SPEED. Shutter speed is a time, and is sometimes called exposure time. Perhaps an even better term would be shutter duration. Most often however, it is called shutter speed, and that term will be used here because most other information you encounter will also use that term.
The selection of shutter speed serves two main purposes in photography: 1) a means of influencing exposure, and 2) a means of controlling motion. Both of these purposes will be discussed in more detail.
Effect of Shutter Speed on Exposure
If you feel you need more information on exposure before proceeding, you can read about exposure here. The more light that strikes the film or sensor, the greater the exposure. The longer the exposure time, the more light strikes the film or sensor. Earlier it was observed that exposure time and shutter speed are interchangeable terms. So the longer the shutter speed, the greater the exposure. That is the effect of shutter speed on exposure. For this reason, shutter speed is sometimes changed when the resulting exposure needs to be changed—sometimes, but not always. Sometimes it is not desirable to control exposure with shutter speed because shutter speed also has an effect on how motion is captured.
Effect of Shutter Speed on Motion
Take a look at the photo below:
This squirrel was in motion during the exposure interval. Specifically, the head and left paw seem to have been moving rapidly, while the rest of the squirrel was fairly still. The photo was taken with a shutter speed of 1/125th of a second. If the photo had been taken with a shutter speed of 1/500th of a second, the squirrel would only have been able to move 25% as far as in that length of time. The result would be that the motion would have been far less apparent.
Naturally, the motion in the above photo was undesirable. But if you were taking a series of photos of someone juggling, it might be nice to have some of them that exemplify the motion of the juggler's arms and the balls moving through the air. It is usually difficult to predict exactly what shutter speed is necessary to perfectly 'freeze' motion. It may be even more difficult to predict what shutter speed is necessary to show a specific amount of motion. Even an experienced photographer is likely to take several photos with widely varying shutter speeds in such a situation. Note that when the shutter speed changes, exposure is affected. For this reason, some other factor must change along with shutter speed so that exposure remains correct. This other factor will usually be either aperture (f-stop) or ISO sensitivity. If you are using your camera in shutter priority mode, one or both of these other factors will be changed for you automatically.
Selecting a Shutter Speed on your Camera
It is worth noting that the numbers you see on your camera when selecting shutter speed are actually reciprocals of the time unless indicated by an apostrophe after the number, in which case they are being expressed in actual seconds. For example, if you select 60, you are selecting 1/60th of a second. If you select 2, you are selecting 1/2 of a second. If you select 2", you are selecting 2 seconds.
A common range of shutter speeds found on a camera with manual controls would be: 2000, 1000, 500, 250, 125, 60, 30, 15, 8, 4, 2, 1, 1", 2", 4", 8", 15", and 30". That range is from fastest to slowest in 1-stop increments. Note that for each stop of decrease, the shutter speed is one‑half of the previous value. Some cameras allow the selection of shutter speeds in 1/2-stop increments or 1/3-stop increments. For this reason, you may see other values as you rotate the dial. If the word stop is unfamiliar, consult the article on exposure. Referring again to the range of values earlier in this paragraph, note that each 1-stop change in shutter speed is a doubling or a halving of the adjacent values. The values 8 and 15 are a slight exception to this which is made to simplify the scale. The article on exposure also explains why the scale is set up the way it is.
It is also worth noting that if you are using any automatic mode on your camera (except shutter priority mode), you will more likely be observing what shutter speed your camera has selected for you, rather than actually selecting it yourself. It is a good idea to observe the settings your camera intends to use before taking the photo (if there is time). Once you understand what effect these settings have, you'll often decide you want to change those settings before taking the photo. One of the most common reasons for deciding to reject your camera's automatically selected shutter speed relates to our next topic, the reciprocal rule.
The Reciprocal Rule
The reciprocal rule applies strictly to hand-held photography, meaning picture-taking with no tripod, no bean bag, no Image Stabilization (Canon), no Vibration Reduction (Nikon). Everyone has shaky hands, some worse than others. When your hands shake, the camera shakes. If the shutter speed is slow enough, this shaking will result in visible motion blur in the resulting photograph. The longer the focal length, the worse this effect is. To grasp this more fully, try this little experiment. Find a laser pointer, and point it at something about three feet away. Hold the beam as steady as you possibly can and estimate how far you think it's moving back and forth. Let's says it's 1/8th of an inch. Now shine the laser pointer on a wall about 20 feet away, and hold it as steady as you can. Odds are the motion of the light beam is a lot more than 1/8th of an inch, probably more like 1 inch.
Unless you intend to use a tripod or other means of stabilizing your camera, the shutter speed you use must satisfy this rule: the shutter speed must be no less than the reciprocal of the focal length (in millimeters). Recall from math class that the reciprocal of a number is the result of dividing 1 by that number. Hence, the reciprocal of 8 is 1/8. Since shutter speeds are mostly fractions of a second, there is no real calculation involved. As an example, if you are using a 50mm lens (or a zoom lens set at 50mm), then you should not attempt to shoot hand-held at a shutter speed slower than 1/50th of a second. Referring to the shutter speed scale mentioned earlier, we see that 1/50th of a second is in between two of our available values (60 and 30). 1/60th is faster than 1/50th, and 1/30th is slower than 1/50th. We should use at least 1/60th or anything faster. Just to be clear, faster is anything with a larger denominator.
Sync speed (sometimes called X-sync) is the fastest shutter speed at which flash photography is possible with a given camera model. To understand why this limitation exists, read this article on the operation of the focal plane shutter, then consider what would happen if all or part of the flash illumination takes place while the shutter is not fully open. Not a pretty picture, pardon the pun. Technically, the concept of sync speed could be left to an article about flash photography. Since the sync speed is a shutter speed, it seemed appropriate to give the subject a cursory treatment.