ENT-flexible endoscopes are important for nasopharyngoscopy examinations. However, hospitals do not apply standardized tests to assess the image quality of the endoscopes they use or consider to purchase. The authors evaluated the Rez Checker Target Nano Matte, a test chart that was designed by Image Science Associates for this purpose. The target was placed in a custom setup designed to position the endoscope tip at a distance of 3.0 cm. The primary metrics, including opto-electronic conversion function, noise, modulation transfer function, and color fidelity, were measured using a custom MATLAB script for the Pentax VNL9-CP, Olympus ENF-V4, and Xion XN HD flexible endoscope. For each of these endoscopes, three examples were measured ten times. In addition, 38 Pentax VNL9-CP endoscopes that are regularly used in their clinic were measured to validate this method for the purpose of quality control. They found that the Rez Checker Target Nano Matte can be used to reliably assess image performance metrics for both procurement and quality control. Whether better image quality also improves the diagnostic accuracy of the ENT-specialist seems plausible, but has to be established.