Application of focal length measurement in parallel light tube optical system

Measurement of focal length and top focal length

Focal length is defined as the distance between the convergence point (focus) of a parallel beam parallel to the optical axis of the optical system and the main point of the image side of the optical system. The focal length of the optical system can also be defined as: when the parallel beam passes through the optical system in the direction with a small Angle from the optical axis, and the distance from the beam convergence point to the optical axis is y ', the focal length of the optical system is calculated by the following formula.