The point in the focal plane intersected by the optical axis of the lens is called what?

• A. Focal Length
• B. Principal Point
• C. Fiducial Mark
• D. Vanishing Point

Answer: (B)

The principal point is the point where the optical axis intersects the image (focal) plane. The optical axis is the central line through the lens, and the focal plane is where the image is formed. This intersection serves as the origin for the image coordinate system and is a fixed reference used in photogrammetry to relate image coordinates to real-world coordinates. Focal length is the distance from the lens to the focal plane along the optical axis, not a point. A vanishing point is where parallel lines appear to meet in the image due to perspective, not a physical point on the image plane. Fiducial marks are reference marks used for orientation, not the axis intersection.