Pan-tilt cameras are widely used in surveillance networks. These cameras are often equipped with telephoto lenses to capture objects at a distance. Such a camera makes full-metric calibration more difficult since the projection with a telephoto lens is close to orthographic. This paper discusses the problems caused by pan-tilt cameras with long focal length and presents a method to improve the calibration accuracy. Experiments show that our method reduces the re-projection errors by an order of magnitude compared to popular homographybased approaches.