The RGB camera has a slightly larger angle of view than the Depth camera
Lots of information on calibrating the depth camera is available on the ROS kinect_node page
From their data, a basic first order approximation for converting the raw 11-bit disparity value to a depth value in centimeters is: 100/(-0.00307 * rawDisparity + 3.33). This approximation is approximately 10 cm off at 4 m away, and less than 2 cm off within 2.5 m. A more dense set of data and second or third order approximation could increase the accuracy maybe by an order of magnitude.
Once you have the distance using the measurement above A good approximation for converting (i, j, z) to (x,y,z) is: x = (i - w / 2) * (z + minDistance) * scaleFactor y = (j - h / 2) * (z + minDistance) * scaleFactor z = z
Where minDistance = -10 and scaleFactor = .0021. These values were found by hand.
To convert the 11-bit disparity value to an 8-bit grayscale value that is fairly linear with respect to distance: (2048 * 256) / (2048 - rawDisparity). Also, background noise can be effectively eliminated by ignoring rawDisparity values above 1023.