Optotune Manually Focus Tunable Lenses are designed to have their lens curvature change between convex, flat, and concave by the rotation of the outer adjustment ring. This allows for the focal length of the lens to be tuned to the precise value required for an optical application. The rotation mechanism is extremely durable, providing a lifecycle greater than 100,000 rotations. Optotune Manually Focus Tunable Lenses provide a simple, cost-effective solution for applications where system parameters are still being decided, such as in system prototyping or R&D applications. These focus tunable lenses can also be used singularly as an adjustable eye relief or as a pair to build a variable beam expander.