American Board of Ophthalmology (ABO) Practice Exam

Which type of lens surface is not accurately measurable with a lens clock?

1. Spherical

2. Cylindrical

3. Aspheric

4. Bifocal

The correct answer is: Aspheric

The aspheric lens surface is not accurately measurable with a lens clock due to its non-uniform curvature. Lens clocks, designed primarily for spherical and cylindrical surfaces, utilize a specific technique that assumes a consistent radius of curvature across the lens surface, which is true for spherical lenses. While cylindrical surfaces may be measurable to some extent, they also have distinct areas of curvature along different axes. Aspheric lenses, on the other hand, are specifically crafted to reduce optical aberrations by featuring a gradual change in curvature across the surface. This complexity means that a lens clock, which relies on measuring a single radius of curvature, cannot provide an accurate representation of the contour of an aspheric lens. The variations in curvature cannot be captured by the simple dial measurements of a lens clock. In contrast, bifocal lenses, though they may have different segments, can still be measured for their spherical components effectively with a lens clock. In summary, the inherent design and purpose of the aspheric lens surface make it incompatible with the measuring capabilities of a lens clock, as they cannot adequately account for the continuous change in curvature that characterizes these lenses.