I want to make structure from motion using the camera from my mobile phone (nokia lumia 625). Now I am at the calibration phase. I tried with 3 sets of images (images with chessboard) and I used the opencv libraries. My problem is that with each set of images I got different results. For example for fx I got 1087, 1093, 1112 and for fy 1085, 1091 and 1110. The center point's coordinates are different as well. These deviations are permissible? Each set of images contains 30 images.