Catadioptric mirrors are widely used in automatic surveillance system. The major drawback of catadioptric mirror is its unequal image resolution. Even double panoramic structure can generate stereo images with single camera system. So two images obtained from double panoramic equidistance catadioptric mirror can be used in finding the depth and height values of object?s points. Equidistance catadioptric mirror can be the solution to this problem. But compared to the single catadioptric mirror, the image size of double panoramic system is relatively small. This leads to the severe accuracy problem in estimation. The exact axial alignment and the exact mount of mirror are the sources that can be avoided but the focal length variation is inevitable. In this paper, the effects of focal length variation on the computation of depth and height of object? point are explained and the effective focal length finding algorithm, using the assumption that the object?s viewing angles are almost same in stereo images, is presented.