Abstract: To meet the demand for high-precision, stepless adjustment of infrared detectors in the front-end optical system of variable F-number systems, establishing an accurate mapping relationship between the target F-number of the detector and the rotation angle of the movable ring is necessary to calculate the actuation quantity according to the desired F-number. For an irregular aperture composed of multiple circular arcs formed by the blade iris structure, the inscribed circle was taken as an approximation of the aperture, and the functional relationship between the F-number and rotation angle of the movable ring was derived based on geometric principles. Polar coordinate modeling was conducted for the actual irregular aperture, and the analytical expression of the aperture area was obtained using the integral method after determining the boundary conditions. The concept of an equivalent F-number was introduced to compare the differences between the two aperture calculation approaches. The experiments analyzed the variation trends of the difference and relative error between the inscribed circle area and the actual aperture area, and the relative error trend of the equivalent F-number was clearly demonstrated. Using a certain type of variable F-number infrared detector developed by the Kunming Institute of Physics as the test object and a video measuring instrument as the measurement tool, the F-number test experiments verified the correctness and effectiveness of the established mapping relationship. A theoretical basis and engineering reference for high-precision F-number switching control of variable-F-number detectors was provided in this study.