What do you call a lens made up of two cylinders of equal but opposite powers?

A lens made up of two cylinders with equal but opposite powers is referred to as a cross cylinder. This configuration creates a lens that can correct astigmatism more effectively by combining the corrective powers of both cylindrical components to achieve the desired optical effect.

In practice, cross-cylinder lenses are utilized in various optical prescriptions to help balance and correct uneven curvature in the eye, thus improving vision clarity. The specific design allows for adjustments to be made in different meridians, enhancing the overall functionality of the lens in treating conditions such as astigmatism.

The other options involve different types of lenses or properties. For example, a cylinder lens typically refers to a single cylindrical lens, while a bifocal lens contains two different optical powers to help with vision at different distances. A concave lens diverges light rays, focusing them away rather than correcting astigmatism. Each has distinct characteristics and applications that set them apart from the cross cylinder design.