When you view one frame of motion-picture film or video containing a moving object, the image is often blurred, because a frame represents a sample of time (in film, a frame is 1/24 of a second long). In that time, a moving object occupies more than one position as it travels across the frame, so it doesn’t appear as a sharp, still object. The faster the object moves, the more it is blurred. The camera shutter angle and shutter phase also affect the appearance of the blur, determining how long the shutter stays open and when the shutter opens relative to the beginning of the frame.

In contrast, in a single frame of a computer-generated animation, you may not be able to tell which objects are moving because all moving objects may appear as sharp and clear as nonmoving objects. Without motion blur, layer animation produces a strobe-like effect of distinct steps instead of an appearance of continuous change. Adding motion blur to layers you animate in After Effects makes motion appear smoother and more natural.

You can use motion blur when you animate a layer—for example, moving a layer of text across the screen. You cannot add motion blur to motion that already exists within a layer, such as live-action video. If you want to smooth live-action video where you assigned a frame rate much lower or higher than the original, use frame blending.

You enable motion blur for each layer individually, and you also determine whether the motion blur is rendered for previews and final rendering. Use the Enable Motion Blur composition switch at the top of the Timeline panel to enable or disable motion blur rendering for previews. Modify the render settings in the Render Queue panel to enable or disable motion blur rendering for final output.

Motion blur slows rendering, so you may want to disable the composition switch while working, and only enable it when you need to see the finished result.

The number of samples that After Effects uses to calculate motion blur adapts for each layer, depending on the motion of that layer. This provides high-quality motion blur without unnecessarily sampling the motion of a slow-moving layer as frequently as that of a fast-moving layer. High sampling rates decrease rendering performance.