A 60-deg-field-of-view optical see-through head-mounted display (HMD) using off-axis optics has been designed for 3-D medical imaging visualization. Two basic on-axis optical design concepts for seethough HMDs are reviewed first, to motivate the design of an off-axis optical form. An off-axis design is then presented. Because HMDs are typically designed from the pupil of the eye to the miniature display, it is common to assess final performance according to the display characteristics. Such analysis, however, does not provide information that is easily translated into task-based performance metric. Therefore, we present an analysis of the performance of the design from a usability viewpoint. For this analysis, the optical system is ray-traced from the display to the eye. Three key measures of performance—accommodation, astigmatism, and chromatic blur—are presented over the field of regard using customized graphical output.