The two skinny lines in the lensometer measure which aspect of a lens?

• A. Cylinder
• B. Axis
• C. Sphere
• D. Prism

Answer: (C)

When using a lensometer, those two slender lines serve as a focus reference for the lens you’re testing. If the lens has only spherical power, both principal meridians bend light by the same amount, so both lines can be brought into sharp focus at the same time and stay aligned. That simultaneous, uniform focus reveals the spherical component. A cylindrical power would affect the meridians differently, so you’d see differences in focusing or orientation when you test the lens, which is how cylinder and axis are determined. Prism is read by lateral shift rather than the focus of these lines. So the two skinny lines indicate the spherical power of the lens.