Optical apparatus (38) includes an electro-optical layer (46), contained within a transparent envelope (43, 44) and having an effective local index of refraction at any given location that is determined by a voltage waveform applied across the electro-optical layer at the location. An array of excitation electrodes (50) is disposed over a surface of the transparent envelope. Control circuitry (42) is configured to apply voltage waveforms to the excitation electrodes so as to generate across at least a part of the active area of the electro-optical layer a phase modulation profile (60, 63, 64, 65, 66, 67, 70) comprising spatially alternating peaks (61) and troughs (62) separated by phase transitions chosen so as to emulate a Fresnel lens. The troughs have respective phase modulation depths that vary by at least one quarter wavelength at a nominal wavelength of 500 nm across at least the part of the active area of the electro-optical layer that emulates the Fresnel lens.