Focal Length

البعد البؤري

Focal length is the directed distance between a principal reference point of an optical system and its corresponding focal point.

Quick Decode

Focal length describes the directed distance from an optical system’s principal reference point to its corresponding focal point. It tells us how the system converges or diverges light; “directed” means the distance has a sign determined by the optical sign convention.

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