What is the measurement unit for lenses' power?

The measurement unit for lenses' power is diopters. This unit quantifies the optical power of a lens, which is defined as the reciprocal of the focal length measured in meters. Specifically, a lens with a power of one diopter has a focal length of one meter.

Understanding diopters is essential in optometry and optics because they convey how strongly a lens converges or diverges light. Positive diopters indicate converging (convex) lenses, while negative diopters denote diverging (concave) lenses. This measurement directly relates to the correction of vision for nearsightedness or farsightedness, making it a fundamental concept in the field of optometry.

The other units—millimeters and inches—are used for physical dimensions and measurements, such as the thickness of a lens or the diameter of a frame but do not relate to the optical power. Percent is also not relevant to lens power; it typically measures a portion of a whole in different contexts and does not pertain to optics.