Xx ′ = ff ′ is one of the formulas to express the positional relationship between objects and images in an ideal optical system. The distance x from the object point to the first focal point, the distance x' from the image point to the second focal point, the first focal length f and the second focal length f' are the position formulas of objects and images with two focal points as reference points. (Descartes' law of symbols is adopted to determine symbols)
Scope of application: spherical refraction under paraxial light condition