I had hoped that by collecting images of a calibration target throughout the field of view at several prescribed distances that I would get very consistent estimates for fx, fy, cx and cy on an ASUS Xtion Pro. However the values I get for successive data sets differ by as much as 30 pixels for focal length and 10 pixels for center point. The per pixel residual is consistently on the order of .05 pixels per observed grid point. I realize this is just a 5 and 3% parameter variance, but this error dominates the absolute triangulation accuracy for a photogrammetric system. Has anyone else found this degree of variability? Can anyone suggest target size and a set of images that insures better estimation accuracy?