Which term describes the distance between the rear nodal point and the focal plane?

• A. Short focal length
• B. Long focal length
• C. Two horizontal dimensions
• D. Focal length

Answer: (D)

Focal length is the distance from the rear nodal point to the focal plane. In a lens system, parallel rays entering the lens are bent to pass through the focal plane, and the back focal length describes how far the rear nodal point sits from that plane. This distance is a fundamental measure of how strongly the lens converges or focuses light, and it directly influences where the image forms and the system’s magnification. The other options either refer to a magnitude label (short versus long) or describe something unrelated (two horizontal dimensions), so they don’t name the specific distance between the rear nodal point and the focal plane.