You must ensure that Motion Blur is enabled in the Physical Camera and adjust the settings in the Render Setup dialog box to modify the effect.

General

Respect Physical Camera Settings

Uses the motion blur parameters defined in the Physical Camera.

Instantaneous Shutter for Motion Vectors

Transform Keys 

Deform Keys

Motion Blur Time Span

Length 

Shutter Position

Start on Frame

Center on Frame

End on Frame

Shutter Behaviour