Affine stereo calibration has been identified as the determination of the plane collineation induced by the plane at infinity. Generally, this so-called infinity homography can be determined provided 3 image correspondences of points at infinity. In this paper, we are first concerned with affine stereo calibration for the case of two cameras with the same intrinsic parameters. It is shown that the affine calibration is possible with one less point at infinity due to the invariance of the intrinsic parameters for the two cameras. Then, we propose two practical methods for affine stereo calibration in the general case. The experimental results both on simulated and real images are presented. The quality of the calibration method is evaluated by the quality of affine reconstruction.