Which term describes the principal point in the focal plane?

• A. Fiducial map
• B. Intervalometer
• C. Principal point
• D. Viewfinder

Answer: (C)

The main idea here is the reference point where the camera’s optical axis meets the image plane, which becomes the origin for image coordinates. This point is called the principal point. It defines where measurements on the image are referenced and is central to calibrating a camera, since pixel positions are measured relative to this origin. In practice, the principal point can be slightly offset from the exact image center due to lens and sensor geometry, and its coordinates (often noted as x0, y0) are part of the camera’s calibration parameters used to map image coordinates to real-world positions.

The other terms describe different things: a fiducial map relates to calibration marks used for reference, an intervalometer is a device that times shutter releases, and a viewfinder is the device you look through to frame the shot. None of these describe the specific point where the optical axis intersects the focal plane.