Optical power

Optical power, also referred to as dioptric, refractive, focal, focusing, or convergence power, quantifies how strongly an optical system bends light. It is mathematically defined as the reciprocal of the system’s focal length, meaning a short focal length yields high optical power. The standard unit is the inverse metre (m⁻¹), which in practice is called the dioptre (symbol dpt or D).

Converging systems, such as convex lenses, possess positive optical power, while diverging systems, like concave lenses, have negative power. A lens’s optical power and focal length are not fixed; they change if the lens is placed in a different refractive medium, such as water. For multiple thin lenses positioned close together, the total optical power is approximately the sum of each lens’s individual power.

Similarly, the power of a single lens can be approximated as the sum of the powers of its two surfaces. This additive property is widely applied in optometry. In the human eye, refractive errors arise when the eye’s total optical power is either too high or too low to focus light precisely on the retina. A myopic (nearsighted) eye has excessive power, focusing light in front of the retina, which is corrected with a minus-power (negative) lens.

A hyperopic (farsighted) eye has insufficient power, so light focuses behind the retina when the eye is relaxed. Astigmatism occurs when the refractive power differs between two perpendicular meridians of the eye, a condition also termed cylindrical power. Anisometropia describes a situation where the two eyes have different refractive powers from one another.