What is the primary use of a focimeter in optics?

The primary use of a focimeter in optics is to measure the power of a lens. A focimeter, also known as a lensmeter, is a specialized instrument that allows opticians to determine the focal strength of a lens, which is expressed in diopters. This measurement is crucial in determining how well a lens will correct a refractive error in a patient's vision.

The focimeter operates by projecting light through the lens and evaluating what happens to that light as it passes through. This provides a direct assessment of the lens's optical power, which is essential for prescribing corrective lenses or verifying the prescription of lenses already made. Understanding the power of a lens is vital in fitting patients with the correct lenses tailored to their individual vision needs.

While the other choices relate to important optical properties, they do not capture the primary function of a focimeter. The device does not measure the refractive index, curvature, or back focal length, as those characteristics are evaluated using different tools and methods in optical practice.