Which statement is false about lens clocks and aspheric lenses?

• A. Lens clocks reliably tell whether the lens is aspheric
• B. Lens clocks do not provide true readings of aspheric lenses
• C. The best clue is the flatter front curve to tell if a lens is aspheric
• D. Lens clocks can measure base curve accurately

Answer: (A)

Lens clocks work by comparing the curvature of a lens surface to a fixed reference curve and convert that into a diopter-like readout. They rely on the assumption that the surface under measurement is spherical, so the reading reflects a single radius. Aspheric lenses, on the other hand, have curvature that changes with radius, so there isn’t one uniform radius to report. Because of that, a lens clock cannot reliably tell you whether a lens is aspheric; the reading is only a local curvature measure and can vary depending on where you place the clock on the lens. So the statement that lens clocks reliably tell whether a lens is aspheric is not correct.

What a lens clock does confirm is local curvature and, for spherical portions, an estimate of surface power; it does not by itself reveal the presence of aspheric design. If you’re looking for a quick cue, a flatter front curve can suggest an aspheric design, but it’s only a clue and not definitive. And yes, a lens clock can be used to assess the base curve by measuring the relevant surface, though accuracy depends on technique and context.