In an automatic focimeter that uses the Hartmann method for measurement, the Hartmann diaphragm can select light and determine the measurement resolution. Its aperture and hole spacing directly affect the quality of the imaging spots on the CCD, resulting in a measurement error in the spot position. In this paper, we established an objective function model according to the arrangement of circular holes on the Hartmann diaphragm and a variable model through the diffraction and interference theory and Matlab simulation. Then, taking the enumeration method for extremum search, we designed an optimization program for the optimal aperture of circular holes. As a result, the optimal aperture of the Hartmann diaphragm in a certain measurement area was obtained. Furthermore, we developed an experimental platform for lens measurement based on the Hartmann method. Comparative experimental results prove that this method can get reliable optimal apertures.