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Blurry Pictures - Part 2: Motion Blur
Learn about Motion Blur and how you can use this knowledge for better shots.
In my previous article, we investigated Camera Shake. Camera shake is always unintentional. It can destroy a good image, but it is easy to avoid.
In this second installment on causes of blurry images, we will investigate motion blur, its causes, and its potential usefulness.
Causes and remedies of motion blur
In the previous article, we learned to adjust exposure time to keep the subject fixed, but what happens if the subject itself moves?
Movement causes motion blur. The camera lens projects an image of our subject onto the sensor or film. We need to ensure that the projected image remains in a fixed position on the sensor. If we wish to keep the subject sharp, we need to adjust exposure time to account for the subject's movement. The closer the subject is, the shorter the exposure time needs to be.
The direction of your subject is also important. A car moving sideways needs a much shorter exposure time to look sharp than a car moving toward or away from you.
Imagine a car on a long road in a great distance speeding toward you. Until the car gets closer, you have a very hard time judging its speed. Its size grows very slowly. A car speeding past you on the other hand, may only be visible for a fraction of a second.
The table below contains exposure time guidelines that will help you to avoid motion blur.
I copied the table from a previous article I wrote on Motion and Shutter Speeds, which explains the subject of avoiding motion blur in more depth and how you may be able to obtain those fast speeds in the first place.
How motion blur looks like
Motion blur looks very different from camera shake. Assuming that we do not have an additional camera shake problem, it is easy to detect.
Motion blur will only affect subjects that move relative to our point of view. In the picture above, the two Segway riders moved relative to my camera view that remained fixed on Alcatraz in the background. The exposure time was too short, which is why the two riders look blurry.
A second characteristic of motion blur is that the blurriness has a direction. The lateral motion of the riders caused horizontal lines to remain relatively sharp, while vertical lines appear smeared. Look at the wrinkles in the jeans of one of the riders or the eyes of the second rider.
Using motion blur to emphasize speed
You can use motion blur to enhance your images by emphasizing movement.
If you move your camera, tracking the moving subject, you can keep the subject sharp while blurring the background. This will improve the sense of speed of the subject, which can make a picture look very dynamic. Please see my article on Motion and Shutter Speeds to learn more about this.
Relative to my camera point of view, the car remains fixed and the landscape around it moves. The motion blur is clearly directional as you can see from the elongated shapes of the foreground and background. This is very different from the out of focus background of the image at the beginning of this article, a desirable effect of the limited depth of field, which will be the subject of another article in this series.
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