1. A method, comprising: a. illuminating a user's eye with an illumination source in a head-worn display; b. capturing an image of the user's eye with an eye camera in the head-worn display, wherein the image includes an eye glint produced by light from the illumination source that is reflected from a surface of the user's eye; c. determining a size of an eye glint in the captured image; and d. identifying a change in focus distance for the user's eye in correspondence with a change in the size of the eye glint.