If you are scrubbing through your timeline looking for a great still to upload on Facebook or to share on Twitter, you may get a bit bummed out when you realize nearly all of your stills are blurry. Yet motion blur is an essential element to a realistic shot with a good amount of motion. It may be undesirable when you are trying to retrieve a still from a 1080p shot with a lot of movement, but it is needed to create an aesthetically pleasing shot (just look at the backlash of The Hobbit), and hey, remember that motion blur even occurs in real life. Stare at a ceiling fan in motion, and then try to follow and individual blade. Motion blur.
The motion blur we see on screen is a natural result of movement that occurs while a camera shutter is open, this causes objects in the image to be recorded at every point of the shutter opening, to the shutter closing.
The movement can be from two sources, the movement of objects such as people running or fast cars, or the camera moving itself. As stated, it does smear a clean image, however it is generally desirable and relaxes the eye.
The idea with motion blur in a realistic visual effects shot is usually to match the amount of blur in the source shot, assuming you have a reference; if you lack visual reference, a camera report can also help you set this correctly. Any moving picture camera has a shutter speed setting that determines the amount of motion blur. This is not the camera’s frame rate, although the shutter does obviously have to be fast enough to accommodate the frame rate. A typical film camera shooting 24fps (frames per second) has a shutter that is open half the time or 1/48 of a second. – Mark Christiansen
Check out the tutorial below from Adobe about applying motion blur within After Effects.
Some footage and 3D renders do not contain enough motion blur, causing visual strobing when played back at normal speed. Chris Meyer demonstrates how to apply the Pixel Motion Blur effect to cure many of these ills.