Troubleshooting Motion Blur

Why don't I see any motion blur?

Make sure that the object with the motion blur property is moving and that the scene's shutter Speed is not set to 0.

For imported scenes, motion blur works only if the scene contains motion vectors. Make sure that the imported file has enough motion vectors to produce motion blur.

Why are the motion blur trails so long?

You most likely have defined too long (slow) a shutter speed. Reduce the shutter's Speed in the scene options (or override it in the pass motion blur options if you don't want to change the global shutter speed).

The motion blur looks grainy

Use Motion Samples to increase the sharpness of the motion blur effect. This parameter is independent of the Sampling Contrast, hence faster to render. Use a low value to achieve a smooth blur, such as below 0.1.