Note that to change the Arnold camera settings, you need to use the Arnold tab in Maya's Attribute Editor for the camera object (make sure you scroll down past the standard Maya 'Camera Attributes' tab to the tab marked Arnold and expand it):
Arnold provides the following camera types:
The perspective, or standard, camera is the most common. Both the perspective and fish-eye cameras provide depth of field and model the effect of altering the camera aperture. The orthographic, cylindrical and spherical camera types enable certain non-perspective views to be created but do not attempt to model any real-world lens effects (they are normally used for creating environment maps).
A scene that demonstrates the above effect can be found here.
Aperture Aspect Ratio
Enables you to turn camera motion blur on or off. 'Use Global Settings' will use the value set in the motion blur tab from the MtoA global render settings. This option can be useful when there are multiple cameras in a scene that may or may not require motion blur. "Use Global Shutter" (enabled by default) signifies that the camera should use the values from the Render Settings, in the motion blur tab. When you want to set a specific shutter in the camera, you can disable "Use Global Shutter" and then it will be taken into account.
The shutter range of the camera can be made smaller than the motion blur range exported from the scene by changing the shutter_start and shutter_end parameters. The value range is between zero and one. The default shutter_start of 0 and shutter_end of 1 means a full camera shutter range equivalent to the motion blur range. A smaller range (0.0-0.5) will decrease the effective shutter aperture time.
The camera Shutter Start and End should be the same value when rendering out a Motion Vector AOV.