With the help of the rapid increment of the resolution of the digital camera and the calculation capability of computer, photogrammetry based on the digital image correlation is widely applied in different research fields. However, the optical dispersion gives rise to the image distortion, when a 2D image is recorded from a 3D surface. This image distortion can contribute an additional error at calculating strain tensor. The main purpose of the work is to provide a calibration method in order to reduce the error caused by the image distortion and to raise the precision at the calculation of strain. In this work the bi-quadratic and bi-cubic interpolation functions are used to establish the relationship between the real coordinate and the distorted image coordinate systems. The results show that the calibration has almost no influence on improving the precision of calculated strain, when the displacement is small. In the case of large displacement, the deviation of strain and displacement can be reduced with the help of the bi-cubic calibration model.