The right angle depends on location to location. Still, as a general rule, if you have the thermal imaging security camera mounted close to the area you want to monitor, you will have to use a small focal length (large opening). If you mount the camera more from the area of interest, you will use a considerable focal length (small space and large zoom). Imagine that a camera with a long focal length is like binoculars. The greater the focal length, the greater the zoom effect.