An augmented reality display unit for use in an augmented reality headset or the like comprising front and rear variable focusing power compression liquid lens assemblies (220, 230) in mutual optical alignment on an optical axis (O), a transparent waveguide display (240) interposed between the front and rear liquid lens assemblies (220, 230) and a selectively operable adjustment mechanism for adjusting the focusing powers of the front and rear compression liquid lens assemblies (220, 230); wherein each of the front and rear compression liquid lens assemblies (220, 230) comprises a fluid-filled envelope (225, 235) having a first wall formed by a distensible elastic membrane (226, 236) that is held under tension around its edge by a peripheral support ring, a second substantially rigid wall (223, 233) formed by or supported on an inner surface of a transparent plate or a hard lens of fixed focusing power, and a collapsible side wall (227, 237), the membrane forming an optical surface of variable optical power, and the adjustment mechanism being arranged to displace the support ring towards or away from the second wall parallel to the optical axis (O) for increasing or decreasing the pressure of the fluid (228, 238) within the envelope thereby to cause the membrane to distend or contract respectively parallel to the optical axis for changing the focusing power of the optical surface of the membrane (226, 236); wherein the second wall (223) of the front compression liquid lens (220) is formed by or supported on a transparent plate or hard lens (222) which is tilted at a first angle to the optical axis to introduce a first amount of prism to a ray of light passing therethrough, and the second wall (233) of the rear compression liquid lens is formed by or supported on a transparent plate or hard lens (232) which is tilted at a second angle to the optical axis to introduce a second amount of prism to a ray of light passing therethrough, wherein the first and second amounts of prism are mutually substantially equal and opposite.