Distance between rear nodal point of lens and focal plane is called what?

• A. Focal Length
• B. Isogonic
• C. Grid
• D. Moire

Answer: (A)

In lens optics, the distance from the rear nodal point to the focal plane is the focal length. The rear nodal point is where rays entering parallel to the axis appear to exit, and the focal plane is where the image forms. The amount of separation between these two points determines how strongly the lens converges light and where the image sits, which is defined as the focal length. In the simple thin-lens model this distance matches the lens’s basic focal length. (In more detailed thick-lens terms you might hear “back focal length,” but for most questions the concept aligns with the focal length.) The other terms—isogonic, grid, and moire—aren’t involved in describing this optical distance.