Paraxial focal length measurement method with a simple apparatus.

In this paper, we realize an accurate and flexible measurement method for paraxial focal length with a simple apparatus. The proposed method utilizes the established measurement principle which is based on the differentiation of the Gaussian optical formula and geometrical relationship of camera-lens system to obtain the focal length of a lens. Some image processing methods are employed to experimentally realize this measurement principle with high accuracy and practicability. Image sharpness evaluation function is utilized to ensure the system conforms to the Gaussian optical formula. System calibration is adopted to compensate for the errors caused by the simple apparatus. In addition, aberration is considered to obtain a paraxial focal length of a lens. Experimental results indicate that repeatability error of measurement system is less than 0.11% for most lenses.