Certified Ophthalmic Assistant Practice Exam

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A lens consisting of two cylinders of equal but opposite powers is known as what?

Cylindrical lens
Cross cylinder
Convex lens
Biconcave lens

The correct answer is: Cross cylinder

A lens that consists of two cylinders with equal but opposite powers is known as a cross cylinder. This type of lens is utilized primarily in refraction testing and astigmatism correction because it can help determine the astigmatic power needed by introducing a specific amount of cylindrical correction across two different meridians. The term "cross cylinder" accurately describes the two cylindrical components that meet at a right angle, allowing ophthalmologists to assess visual acuity and refine prescriptions effectively. This distinguishes it from cylindrical lenses that have just a single cylindrical power, and from convex or biconcave lenses, which are designed for other refractive purposes without the specific dual meridian focus characteristic of a cross cylinder.