A magnetic resonance imaging system (10) includes a transmit coil (22) and one or more receive coils (32). The transmit coil includes one or more circuit segments (44, 44, 80, 90) including a light-sensitive metal-insulator-semiconductor capacitor (50) which is connected by an optic fiber to one of a plurality of variable light sources (68). In the set-up mode, the transmit coil transmits RF pulses into an examination region (14). A plurality of the receive coils are disposed around the imaging region. The tuning processor (60) analyzes the received RF fields from around the imaging region and determines adjustments to the amount of light transmitted to each light-sensitive capacitor to shim or tune the transmit coil to optimize RF field homogeneity. Further, the receive coils (32) include a light-sensitive capacitor whose illumination is changed during RF transmission to detune the receive coil.