Shooting shallow depth of field with minimal grain

Hey all,

This is quite a basic question…please bear with me.

I'm using a Sony AX2000 HD Camcorder to shoot footage both indoors and outdoors.   My problem is that when I attempt to get a shallow depth of field by adjusting the Iris, the result gets some grain in the image.

I want to know what I should do to keep the shallow depth of field while lowering the grain.  Should I set the ISO lower?  Change the arpeture?  Adjust the shutter speed?    Maybe I'm a little unclear about how each relates to the grain in the video.

Thanks!