Shutter speed, how it works

In the last entry I said that there were three basic things that determined exposure: shutter speed, aperture, and ISO. Shutter speed is the first thing we’ll discuss, since it’s easy to understand and its effects are obvious and intuitive.

As I explained in a six-year-olds view of your camera, there’s a sensor at the back of the camera that records light. This sensor is jam-packed with light-sensitive gnomes who do nothing but wait for light to hit them and then record whatever light they see. Most of the time these gnomes are very bored, because there’s a curtain in front of them that blocks out all the light. That curtain is called the shutter.

When you press the button on your camera to take a picture, the shutter opens briefly (or usually briefly, at least) and then closes again. While the shutter is open, light falls on the sensor gnomes and they excitedly write down what they see. In scientific terms we call this taking a picture.

If the shutter is open for a long time, a lot of light hits the sensor gnomes. If it’s only open briefly, the gnomes get correspondingly less light. The amount of time the shutter is open is called, amazingly enough, the shutter speed.

Shutter speed is expressed in fractions of a second, and is most commonly in the range of about 1/30 sec to 1/500 sec, though it can range from many seconds up to the maximum speed of your camera– frequently 1/2000 or 1/4000 of a second. Your camera may identify the shutter speed that way, but more commonly it will only use the lower number. On my Canon, shutter speeds are displayed as 30, 100, 500, etc. Those mean 1/30, 1/100, and 1/500 second respectively. For slower shutter speeds, my Canon uses a slightly different display: 1/4 sec is displayed as 4, but then it goes to 0”3, which means .3 seconds, then 0”4, 0”5, 0”6, 0”8, 1”, etc. 1” is one second, of course. From there it keeps climbing up to 30”, which is 30 seconds.

The neat thing about shutter speed is that it can be used to display motion in your photographs. If you use a really fast shutter speed, you freeze whatever action is happening. Using a slow shutter speed will let you capture motion blur.

In order to illustrate this, I stuck my camera on a tripod and took a series of photos. I used a glow poi (a light-up ball on a string) and spun it in circles while taking a sequence of pictures at different shutter speeds. In the first picture, taken at 1/8 of a second, you can clearly see the motion of the ball as it sweeps around its arc.

At 1/15 of a second, there’s still motion but less of the arc is visible. At 1/125 sec the ball just looks like a blur, and finally at 1/500 sec the ball is completely frozen in its tracks.

That’s how shutter speed works, in a nutshell. Next, we’ll see some real-world examples of why you might want to use a specific shutter speed.

Next lesson: Shutter speed, why you care