Determination of the paraxial focal length according to its definition by the German DIN standard in measurements

In the world of optics, many different definitions about the focal length exist. The German DIN standard basically differentiates between two focal length definitions: the real focal length and the paraxial focal length. While defining the real focal length as the focal length of a finite wavelength range together with a finite aperture and giving a measurement procedure for it, the DIN standard doesn’t say how to measure the paraxial focal length. It solely defines it as the focal length of a certain wavelength with the aperture at the transition to zero. While investigating focal length measurement procedures using Zernike polynomials, we’ve developed a procedure to retrieve the paraxial focal length as it is defined by the German DIN standard using experimental raytracing. Simulations and measurements performed with the developed procedure using experimental raytracing and a Shack-Hartmann-Sensor have been evaluated and compared.