Understanding Shutter Speed
In an earlier blog post overview on Understanding Exposure, I talked about the relationship of the "Three Speeds", shutter speed, lens aperture, and sensor speed (ISO). In this post I want to discuss the first, and probably most obvious, of these, shutter speed. A camera shutter controls the amount of light allowed to hit the sensor by controlling the time interval it is remains open. For instance, a shutter that remains open for 1/60th of a second, it allows twice as much light to hit the sensor as a shutter set to 1/125th of a second. Any time this doubling progression occurs, we call it a "full stop" increment.
A list of the full stop shutter increments goes: 1sec, 1/2sec, 1/4sec, 1/8sec, 1/15sec, 1/30sec, 1/60sec, 1/125sec, 1/250sec, 1/500sec, 1/1000sec, and so on. Some modern cameras have shutter speeds as fast as 1/8000 sec. And on most modern cameras shutter speeds often progress in longer (i.e. "slower") increments from 1 second down to 30 seconds. After 30 seconds you can use the "Bulb" feature, which means that the shutter will remain open for a long as you keep the shutter button or cable release pressed. So in "bulb" mode, you control the actual amount of time the shutter remains open. This is handy for photographing subjects like fireworks, as we shall see later.
The most obvious use of shutter speed is freeze the action in a scene. A shutter speed that is too slow will allow the subject to appear as a blur in the photograph. There are two possible causes of blur: the subject being photographed may be moving too fast for the shutter speed being used, or camera shake caused by unsteadiness in holding the camera by hand. The photos below illustrate blur caused by camera shake.
Generally speaking, a camera should not be hand held with a shutter speed below 1/60th of a second with a normal focal length or less, and even then special care should be taken to steady it. The actual usable speed is also relative to the focal length being used. Telephoto lenses magnify movement so that 1/60sec with a 50mm lens is equivalent to using 1/125sec with a 100mm lens, the same as 1/250sec with a 200mm lens. In addition, most modern DSLR lenses and cameras are equipped with vibration reduction systems that can extend the usable slow shutter speeds. For instance, hand holding a lens with a vibration reduction rating of 2 means that it can be safely hand held at two shutter speeds less than normal.
Using blur creatively:
Blur can also be caused by a shutter speed that is too slow to stop the action of a moving subject. The photo below of the cowboy riding through the falling snow shows the result of a double blur effect from both camera motion and subject motion.
Another use of slow shutter speed is to create motion in flowing water.
The shutter speed needed to freeze the action of a moving subject depends upon three things: the speed of the subject, the focal length of the lens, and the direction of the motion relative to the camera.
While a shutter speed of 1/250sec may be sufficient to stop the action of a person walking, it might take upwards of 1/2000sec to stop the action of a fast moving race car. The actual speed needed to stop the action also depends on the focal length. Telephoto lenses require higher shutter speed to stop action than do wide angle lenses. Finally, a subject moving across the picture frame also requires a higher speed to stop its action than a subject moving towards the frame. Let's look at some examples.
Here the action of the jumping athlete was frozen in mid-air with a shutter speed of 1/1000sec . The photo was taken with a 400mm telephoto lens with the subject moving towards the camera.
Choosing a shutter speed is not always arbitrary. Often the correct choice can enhance a subject by either freezing the action or by allowing its motion to blur. It is best to experiment a bit with various speeds to gain some experience of what speed works best each situation and with the various focal lengths.
|A shutter speed of 1.3sec blurred the falling in much the same way as it did in the waterfall samples earlier.|
|For these bursts of fireworks the shutter was left open for a full 4seconds. This allowed the bursting fireworks to "paint" themselves against the black sky.|