A 1D steeper K makes a tear lens of what power?

A 1D steeper K makes a tear lens of what power?

• A. -1 D
• B. +2 D
• C. +1 D
• D. 0 D

Answer: (C)

The tear lens power comes from the thin tear film between the cornea and the back surface of the rigid lens. It acts as a small lens in series with the cornea, and its power is roughly the difference between the corneal power and the back-surface power of the lens. If the cornea becomes 1 diopter steeper, its refractive power increases by about 1 D. With the back surface of a typical GP lens not adding a large opposing power, the tear layer ends up providing about a +1 D power to balance that change. So the tear lens is about +1 D. A flatter cornea would trend toward a zero or negative tear lens, depending on the lens fit, but the 1 D steeper keratometry yields approximately +1 D for the tear lens.

The tear lens power comes from the thin tear film between the cornea and the back surface of the rigid lens. It acts as a small lens in series with the cornea, and its power is roughly the difference between the corneal power and the back-surface power of the lens. If the cornea becomes 1 diopter steeper, its refractive power increases by about 1 D. With the back surface of a typical GP lens not adding a large opposing power, the tear layer ends up providing about a +1 D power to balance that change. So the tear lens is about +1 D. A flatter cornea would trend toward a zero or negative tear lens, depending on the lens fit, but the 1 D steeper keratometry yields approximately +1 D for the tear lens.