A lens clock is used to measure which of the following?

• A. The frame width across the front
• B. The surface powers and base curve of the lens
• C. The pupillary distance
• D. The lens thickness

Answer: (B)

A lens clock measures the curvature of a lens surface, expressed as the surface power in diopters. This surface power is what defines the base curve—the curvature of the lens’s back surface. By placing the clock on the lens surface, you get a reading that reflects how curved that surface is, which is directly used to determine the base curve. The other items—frame width across the front, pupillary distance, and lens thickness—require different tools or measurements and aren’t what a lens clock provides.