To input an accurate magnification image, calibration of the focal length is needed in an active camera system which can control the zoom, focus, iris, and viewing angle. We show that in a calibration with a pinhole camera model, the arrangement of the measurement points influences accuracy and good camera parameter estimation is difficult by using a single target. In addition, we show that by calibrating with the zoom lens camera model we propose, good camera parameter estimation can be achieved even when the number of the measurement points on a single plane is small and the error of the image coordinates is large.