What is the term for a lens consisting of two cylinders of equal but opposite powers?

The term for a lens that is made up of two cylinders with equal but opposite powers is known as a cross cylinder. This type of lens is specifically designed to correct for astigmatism by providing two different cylindrical powers oriented at 90 degrees to each other. The equal but opposite powers effectively neutralize the astigmatic error, allowing for a clearer vision in all meridians of the eye.

Cross cylinders are particularly useful in refraction tests and are often employed in contact lens fitting and eyeglass prescriptions. They allow optometrists to fine-tune the prescription needed to address astigmatism by testing different orientations and combinations until the best visual acuity is achieved. This precision is what distinguishes cross cylinders from other lens types, emphasizing their unique role in vision correction involving astigmatism.