Primary & Secondary Focal Points

البؤرتان الأولية والثانوية

The primary focal length f_1 is the object distance for which the conjugate image point is at optical infinity, or alternatively it is the directed distance from the interface to F_1. The secondary focal length is the image distance for incident plane waves.

Quick Decode

The primary focal point (F₁) is the object point whose light emerges from an optical system as parallel rays, so its image is at optical infinity. The secondary focal point (F₂) is the image point formed by incoming rays parallel to the optical axis. Their focal lengths describe the directed distances to these points from the appropriate reference surface or plane.

Still confused?

Decode it differently.

Ask Decode

Ask about this concept and get an answer grounded in its Decode content and references.

References (1)
  1. Keating, M. P. Geometric, Physical, and Visual Optics (2nd ed., Chapter 7).