Previously, we discussed the concept of exposure, and it’s three components – aperture, shutter speed, and ISO. Perhaps the easiest component for beginners to understand is shutter speed, so we’ll begin there.
Shutter speed is the simple function of how long you open the shutter of your camera to expose the film or sensor to the light of the scene that you are trying to capture. This measure is measured as fractions of a second (ex. 1/200 sec, or one two-hundredth of a second). The longer you leave the shutter open, the more light you expose the sensor to. The shorter you leave the shutter open, the less light you expose the sensor to. It’s that simple. With regard to the greater concept of exposure, the amount of time you leave the shutter open is directly proportional to the amount of light you expose the film or sensor to.
What do I mean by directly proportional? If you leave the shutter open for 1/100 sec, you expose the sensor to a certain amount of light. If you reduce the shutter speed to 1/200 sec, you’re leaving the shutter open for half of the time as 1/100 sec, and thereby reducing the light exposed to the sensor by half as well. If you increase the shutter speed to 1/50 sec, you’re leaving the shutter open twice as long, and exposing the sensor to twice as much light.
A new item of terminology I’ll introduce you to now is a stop. Exposure is usually discussed, and adjusted, in terms of stops, so it’s important to understand the term and it’s significance. A stop is used to describe the relative level of light by a factor of 2.
In simpler terms, if you double the amount of light, you’ve increased the exposure by 1 stop. If you halve the amount of light, you’ve decreased the exposure by 1 stop. One important point to keep in mind is that each incremental stop is a factor of 2, so while increasing by 1 stop is double the amount of light, increasing by 2 stops is four times the amount of light, and increasing by 3 stops is eight times the amount of light. Likewise, decreasing exposure by 1 stop is 1/2 the amount of light, decreasing by 2 stops is 1/4 the amount of light, and decreasing by 3 stops is 1/8 the amount of light. And so on…
In addition to simply adjusting the amount of light the sensor is exposed to, understanding the effect of shutter speed on exposure and your final images will allow you to make adjustments for artistic effect. Using a faster shutter speed will leave the sensor exposed for only a brief period of time, so objects will appear frozen, with no motion.
While using a fast shutter speed will freeze objects in place, using a slow shutter speed will cause objects in motion to blur which, when used effectively, can imply a sense of motion or activity in your still photography. Cars turn into streaks of light. Clouds take on an ethereal quality as they streak across the sky. Flowing water takes on an eerily smooth, cloudy quality.
There is no “right” shutter speed for a given photograph, as you can see in the examples above. The “right” one is the one that best conveys the photographer’s artistic vision for the scene they’re presenting in the capture. Whether you’re looking to stop motion or prolong it in your images is at your discretion. Now that you understand how shutter speed can be utilized and controlled, we’ll next examine the next component of exposure – aperture.