<p>Augmented reality in the operating room enhances surgical precision. Merging information from preoperative 3D imaging into the real-time video during surgery allows for better visualization of critical structures. Displaying an augmented real-time video on a screen requires a 2D/3D to 2D image registration as a preliminary step. An accurate error estimation is crucial for evaluating the system’s performance. A reprojection error estimation has been commonly calculated based on a pinhole camera model, which does not accurately represent the optical properties of an endoscope or an operative microscope. In our work, a system for registering a CT-scan to a 2D-otoendoscopic video was evaluated. This system was designed for use in an augmented reality setup for ear surgery. Evaluation was performed using the target registration reprojection error measurement. A comparison between the pinhole camera model and the thin-lens model was conducted. Additionally, a quantification formula for the target registration reprojection error was established and the parameters affecting it were analyzed. Specifically, the target’s distance from the camera lens and its angle of alignment with the optical axis were assessed. Five human ear resin models bearing spherical Markers with their corresponding CT-scans were used. The estimated error with the thin-lens model was 0.65 ± 0.52 mm versus 0.79 ± 0.60 mm with the pinhole model (18% of difference). Target registration reprojection error tended to be correlated to the distance from the camera lens. In conclusion, both the target-lens distance and the target alignment influence the target registration reprojection error measurement. The choice of the optical model affects significantly the error estimation, and the thin-lens model should be preferred in endoscopic applications. Based on the results, the tested augmented reality system is compatible with ear surgery.</p>