If a camera is mounted 40 feet (12 meters) above the ground on the outside wall of a building and is focused on an entry gate that is 30 feet (9 meters) from the building, what is the focal length of this application?
The distance from the camera to the gate is the straight-line distance, which must be calculated using the Pythagorean theorem since the camera is mounted above ground and the gate is a horizontal distance away.
Using the formula:
√(height² + horizontal distance²)
= √(40² + 30²)
= √(1600 + 900)
= √2500 = 50 feet
So, the focal length must accommodate a 50-foot (15-meter) distance.
[References:, PSP Study Guide – CCTV Design Principles, POA Manual – Video Surveillance Planning and Optics]
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