When we see lines drawn radiating from the back of a car, a cat, or a comic strip character, we instinctively know that the subject is supposed to be in motion. Those lines represent motion blur, which is actually a photographic mistake caused by using a slow shutter speed on a fast subject. The image's subject appears totally or partially blurred against the background because the subject actually traveled some distance during the fraction of a second that the camera shutter was open.
In the early days of photography, motion blur was a common occurrence, primarily because shutter speeds were slow, and film sensitivity was not very great. Today, motion blur is unusual, unless the photographer is capturing the subject this way on purpose by using the least sensitive film available or by using a small lens opening and a correspondingly slower shutter. If you want to try to approximate the effect of motion blur, Photoshop gives you a tool that can do it.
The Motion Blur (Filter>Blur>Motion Blur) filter can add the appearance of motion to a stationary object by placing a directional blur for a predetermined distance. In the Motion Blur dialog box, shown in Figure 15.14, you can set both the distance and direction of the blur according to how fast and in what direction you want the object to appear to be traveling. The distance sets how much of a blur is applied—or how far the original image is "moved." The angle sets the direction of the blur. To adjust, drag the Radius slider or enter precise values into the field next to it. The trick, however, is to select the right area to which to apply Motion Blur. To get a convincing blur, you need to blur the space where the object theoretically was, as well as to where it theoretically has moved.
Using the Motion Blur filter is tricky at best.
The Motion Blur filter doesn't do much for most photos. After all, the blur caused by the camera shaking is the kind of thing we usually try to avoid—not add. But, for some special effects, and for doing tricks with type, it has interesting possibilities. Figure 15.15 shows one possible use. First, I rendered the type and applied the Perspective transformation to give it some depth. Adding Motion Blur lets me run faster, but I could even take this further.
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