For most imaging cameras, it is desirable that the sensor plane be perpendicular to the optical axis. Such an orientation ensures that the imaging configuration is perspective and planar scene objects perpendicular to the optical axis can be focused in their entirety. In this paper, we have presented an image processing based method to estimate and subsequently correct the sensor tilt with precision. We propose to measure the tilt by measuring the variation of defocus in an image of a planar calibration chart placed perpendicular to the optical axis. We have shown that the proposed defocus based method is inherently more accurate than geometry-based techniques, which estimate tilt by measuring the deviation in the geometry of a scene pattern. We have analyzed the sensitivity of the tilt estimates to errors in the experimental setup and have shown that the proposed technique is quite robust to errors even as large as 1 degree in the orientation of the calibration chart.