Wednesday, July 18, 2012

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.  

Stopping action:

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. 

These photos were taken with a 200mm telephoto focal length lens on a hand held camera.  On the left a shutter speed of 1/500th second was sufficiently fast to result in a sharp image, whereas on the right a speed of 1/30th second caused blurring in the leaves. 
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.  

The camera was intentionally panned to the left at a speed matching the moving cowboy.  A slow shutter speed of 1/25sec  resulted in the falling snow and moving legs of the horse being blurred and streaked.  Because the camera was moving at the same rate at the cowboy he remained relatively sharp.

This photo shows the result of subject blur.  The camera was on a tripod for this nighttime photo of Trafalgar Square, London.  A shutter speed of 1/4sec was not fast enough to freeze the action of the moving traffic resulting in buses blurring as they went by.  The rest of the scene is sharp because the camera was on a tripod.
Here is another example of the blurred subject technique done for effect.  The exposure was a full 3 seconds.  Such a long exposure allowed the lights from the passing traffic to create bright blurring streaks as it passed by.  The stationary scene of the London Parliament buildings remain sharp because the camera was on a tripod.
Another use of slow shutter speed is to create motion in flowing water.

The waterfall photo on the left was taken with a shutter speed of 1/100sec.  At such a high speed the falling motion of the water is stopped.  For the photo on the right a shutter speed of 1/2sec was used to allow the flowing water to blur in the frame and give more of a feeling of the actual waterfall.  The slower speed required the camera to be on a tripod during the exposure.
Freezing action:

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.

This photo shows a ball player sliding into a plate.  His direction is directly into the camera and the shutter speed of 1/1250sec was sufficient to freeze his action.  The baseball, however, is moving across the picture frame so that even at such a high shutter speed it still shows some motion blur.
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.
In this example the subject is moving across the picture frame.  The picture was taken with a 35mm wide angle lens and shutter speed of 1/640sec.   The wider angle lens helped to freeze the action, but even at a relatively high speed of 1/640sec there is still some traces of blur in the hands and feet.  This is caused mostly to by the direction of motion across the picture frame.

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.