In this paper, we document and demonstrate a means of directly measuring the entrance-pupil diameter of an unknown camera system. It is common practice to make direct measurements of a lens assembly, either by imaging with a second camera and counting pixels or using an external measurement instrument, such as a traveling microscope. These procedures favor back-illumination of a freestanding lens, however, which is not always feasible in a closed system. Here, we describe an alternative measurement procedure that utilizes a point source at the focus of an off-axis Newtonian collimator to generate collimated light; this allows us to place a beam block between the incident light and the infinite-conjugate system under test, where the normalized power received depends solely on the scan distance across the block and the entrance-pupil diameter. Thus, by measuring the normalized power focused in the image plane as a function of scan distance, we can calculate the system's entrance-pupil diameter with a high degree of confidence. We demonstrate this concept both through numerical simulation and laboratory testing over a range of entrance-pupil diameters. We further show a comparison with the traveling-microscope technique in terms of accuracy and ease of use. These methods improve our ability to update existing system models with objective measurements, as entrance-pupil diameter factors into many critical performance metrics, including optical cross section and modulation transfer function. The tools developed in support of this reproducible research effort are available on GitHub (https://github.com/derekburrell/camera-evaluation). (c) 2025 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.
