According to ANSI Standards, what does the "cylinder" define?

The "cylinder" in optical terminology, particularly within the context of eyeglasses and contact lenses, specifically refers to the refractive power needed to correct astigmatism. In ANSI Standards, the correct interpretation of what the cylinder defines is not that it "contains no refractive power." Instead, the cylinder represents the part of the lens that corrects for astigmatism, providing the necessary power at a specific axis to focus light properly on the retina.

The presence of aberration, the spherical power component, or bicentric grinding are not definitions of what the cylinder specifically represents. Instead, they pertain to additional aspects of lens design and functionality. Understanding this distinction is key for anyone involved in ophthalmic dispensing, as it relates to the accurate fitting and prescription of lenses to ensure optimal vision correction.