When predicting the required GP lens power, tear layer power should be included.

• A. Tear layer power is ignored.
• B. Tear layer power is the sole determinant.
• C. Tear layer power is optional.
• D. Tear layer power must be included.

Answer: (D)

When predicting GP lens power, the tear layer between the cornea and the back surface of the lens acts like a thin lens of its own. This tear film changes the total refractive power delivered to the retina, so the final eye power with a GP lens in place is the combination of the corneal power, the tear-lens power, and the lens’ own back-surface power. Because the tear layer can vary with blink, tear thickness, and lens fit, it reliably influences the needed lens power. If you skip accounting for tear layer power, your predicted lens power will be off, leading to under- or over-correction after a fit. So tear layer power must be included.