A mechanism allows for changing the optical strength of a liquid-filled lens having a cavity filled with fluid. A reservoir contains additional fluid and is in fluid communication with the cavity, and the mechanism draws fluid from the reservoir into the cavity or pulls fluid from the cavity back into the reservoir, whereby a change in the amount of fluid within the lens cavity changes the lens's optical strength. A membrane seals the reservoir, and a plunger impinges upon the membrane. A movement device, such as a barrel screw or lever, moves the plunger in a controlled manner toward the membrane to increase pressure within the reservoir and thereby force fluid out of the reservoir into the lens cavity, and away from the membrane to decrease pressure within the reservoir and thereby withdraw fluid from the lens cavity and into the reservoir.