<P> Convex lenses have positive dioptric value and are generally used to correct hyperopia (farsightedness) or to allow people with presbyopia (the limited accommodation of advancing age) to read at close range . Concave lenses have negative dioptric value and generally correct myopia (nearsightedness). Typical glasses for mild myopia will have a power of − 1.00 to − 3.00 dioptres, while over the counter reading glasses will be rated at + 1.00 to + 3.00 dioptres . Optometrists usually measure refractive error using lenses graded in steps of 0.25 dioptres . </P> <Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2015) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2015) (Learn how and when to remove this template message) </Td> </Tr> <P> The dioptre can also be used as a measurement of curvature equal to the reciprocal of the radius measured in metres . For example, a circle with a radius of 1 / 2 metre has a curvature of 2 dioptres . If the curvature of a surface of a lens is C and the index of refraction is n, the optical power is φ = (n − 1) C. If both surfaces of the lens are curved, consider their curvatures as positive toward the lens and add them . This will give approximately the right result, as long as the thickness of the lens is much less than the radius of curvature of one of the surfaces . For a mirror the optical power is φ = 2C . </P>

Define the si unit of power of lens