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Shutter Speed, Exposure and Motion

February 16, 2010 Leave a comment

To correctly expose a photograph so that it is neither too dark or too light you need to control the amount of light that reaches the sensor in your camera.

There are two main things in your camera that you can control that affect the exposure of your photograph.

1: The size of your Aperture, which is the size of the Iris of your camera (this also controls your depth of field)

2: Shutter Speed, the length of time you allow the shutter in your camera to remain open, (this also controls motion blur)

Most Digital SLR cameras have a Focal-Plane shutter.  This consists of a pair of curtains usually located just in front of the sensor.  During the exposure the curtains open to form a slit that moves across the light sensitive surface.  The size of the slit is adjustable: the wider the slit the longer the exposure and the more light that reaches the sensor.

The slit moves across the sensor in much the same way that a copy machine or a scanner works.

This slit is your Shutter and the amount of time it remains open is your Shutter Speed.

Depending on your camera, it will display the Shutter Speed in seconds or fractions of seconds.

Depending on how you want your photograph to turn out, you will have to decide what Shutter Speed to use.

For example, a fast Shutter Speed of 1/250 of a second can capture motion well.  This means that your shutter remained open only for 1/250 of a second.  The photograph will freeze motion.

Where as a slow shutter speed 1/8th of a second will show most moving obtions blurry, otherwise known as motion blur.

Typical shutter speeds are:

2 sec, 1 sec, 1/2 sec, 1/4 sec, 1/8 sec, 1/15 sec, 1/30 sec, 1/60 sec, 1/125 sec, 1/250 sec, 1/500 sec, 1/1000 sec

However, some cameras only display the bottom fraction of the shutter speed and seconds are then displayed with quotation marks following them:

2″, 1″, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000

Below you will see some different ways that a camera will display the same shutter speed of 1/250 of a second.

There are a few ways to use Shutter Speed to show motion in your camera.  You can use a fast shutter speed to freeze time.  Typically 1/250 or 250th of a second is a fast enough shutter speed to freeze the motion of humans and most objects.  However, this is all relative to what you are photographing.  If you want to freeze the wings of a hummingbird you may have to go as fast as 1/8000th of a second to freeze its flapping wings.

A less drastic example of a bumper car is below.

The image to the right was taken at a 1/500 of a second shutter speed.  The motion of the bumper car has been frozen in time.  Notice how there is no motion blur.

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While this image was taken with the camera still but at 1/30th of a second.  The bumper car is just a blurr.  This is motion blurr.

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There are two ways you can take this image at 1/30 of a second shutter speed and show the subject (bumper car) sharp.  One way is to pan the camera with the motion of the subject.  This technique has the added effect that the background will be blurry due to motion blur, but your subject will appear frozen in time.  The key to making this or any action shot work is to anticipate the motion of your subject and follow it.

Think about what your subject might do next instead of focusing on what has already happened.

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One other way that this photograph can be taken at 1/30 of a second is to photograph it head on.  Because in this photograph the bumper car is moving straight at the camera, not enough of it crossed the recording surface to blur.  Whereas the bumper car on the lower left is going across the photograph and becomes blurred.

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One last note on Shutter Speed.  The length of your Focal Lens also affects how long of a Shutter Speed you need to capture even stationary objects without motion blur when you are hand holding your camera.  Typically 1/60 of a second is a good enough shutter speed for the typical lens that comes with most DSLR’s (18mm-55mm).  Though use at least 1/125 whenver possible.  Most Digital SLR’s come with an 18-55mm focal length lens.  Longer your focal length the more magnified an object is and it makes the object appear closer to the camera and the faster your shutter speed needs to be to avoid motion blur.   The rule of thumb is, if the ration of your focal length divided by your shutter speed is less than 1 than you are for the most part safe.  For example if you have a 200mm lens and are using a 60th of a second shutter speed, chances are that if you are hand holding your camera the image will be blurred.  If you are using a 200mm lens you would want to use at least a 250th of a second (1/250) shutter speed.