Bodkin Arrow

I am attempting to convert pixel to real world, there are two steps in this process, as per my understanding there are two steps to this namely, application of camera matrix, followed by correction of lens distortion. http://zone.ni.com/reference/en-XX/help/370281AG-01/nivisionconcepts/spatial_calibration_indepth/ What is the order of these steps? What the 'r' value in the distortion correction equation. ? Want to get a direct equation for the conversion of pixel to real world. Please help me with this.