We present Focal, an eye-tracking musical expression controller which allows hands-free control over audio effects and synthesis parameters during peformance. A see-through head-mounted display projects virtual dials and switches into the visual field. The performer controls these with a single expression pedal, switching context by glancing at the object they wish to control. This simple interface allows for minimal physical disturbance to the instrumental musician, whilst enabling the control of many parameters otherwise only achievable with multiple foot pedalboards. We describe the development of the system, including the construction of the eye-tracking display, and the design of the musical interface. We also present a comparison of a performance between Focal and conventional controllers.