What Causes a Motion Blur on a Camera?
Motion blur can be created by a camera in a few different ways, but they all relate to shutter speed. Shutter speed controls the amount of time light is being let into the camera. Blur can be accidental or intentional, subtle or overt. Understanding the conditions under which your camera creates blur can help you control it and use it for possible creative effects.
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Low Light
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For a point-and-shoot camera that does not have manual controls, motion blur often happens under low-light conditions. This is because the camera is trying to gather as much light as possible to make an exposure, and increasing the length of the shutter speed is one way to do this. The subject moves during this long exposure, creating a "ghost image" or blur. Indoor lamp lighting, although it may not seem like low light to the eye, does not provide enough illumination for film, so blur is often created under these conditions. Using a flash creates enough light to make photos sharper.
Hand Holding the Camera
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Even under well-lit conditions, motion blur can occur just by the simple fact that the camera is being held by hand. Even though you may feel you are standing still, subtle body motions have an effect on sharpness. This tends to happen somewhere around 1/30 of a second and slower for a normal 50 mm lens. You can brace yourself against a wall, place the camera on a flat surface to reduce blur or use a flash.
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Slow Shutter Speed by Choice
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You can purposely choose to use a slow shutter speed in combination with a tripod to use motion blur to your advantage. This is how photographs of a moving car's headlights in a glowing line are made; the tripod holds the camera still to keep stationary objects sharp, but a slow shutter speed of a second or longer blurs the motion of the headlights.
Long Lenses
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Long lenses can easily create blur if they are not held very still. A 300 mm lens, for example, must be used at a shutter speed 1/500 of a second to ensure there will be no blur. To avoid blur, either use a tripod for your camera or a smaller physical support just for the long lens, which tends to be heavy and difficult to control.
Subject Distance to Camera
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Blur is created when objects move rapidly across the camera's viewfinder. A person who is running very fast 15 feet away from the camera may easily be captured without blur at a shutter speed of 1/60 of a second, for example. If that person runs at the same speed only a foot away from the camera, they will have traveled across the viewfinder much more quickly, and are more likely to appear blurred in the resulting photograph.
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