Given refraction -3.00 -1.25 x 180, Ks 44/45 @090, BCR 7.58 mm, what would be the predicted GP lens power?

• A. -3.00 D
• B. -3.50 D
• C. +3.50 D
• D. -4.00 D

Answer: (B)

When predicting a GP lens power, we convert the manifest refraction into a single spherical value so a spherical GP can be used unless a toric lens is required. This is done by taking the spherical component and adding half of the cylinder: SE = sphere + (cylinder/2). Here the sphere is -3.00 D and the cylinder is -1.25 D, so SE = -3.00 + (-1.25)/2 = -3.625 D. Lens powers come in quarter-diopter steps, and the closest standard spherical GP power is typically -3.50 D (sometimes -3.75 D, but -3.50 is the nearer common value). The cylinder axis doesn’t affect the spherical equivalent, so a spherical GP lens is chosen with this power, while the keratometry (Ks about 44/45 at 090) and the back-contact radius (BCR 7.58 mm) guide the base curve fit, not the power. So the predicted GP lens power is about -3.50 D.