What is the best vertex distance to refract at?

• A. 8-10 mm
• B. 12-15 mm
• C. 15-18 mm
• D. 20 mm

Answer: (B)

Vertex distance is the space between the cornea and the back surface of the glasses during refraction. The eye’s focal power interacts with that distance, so the effective lens power you ask the eye to accept changes as you move the lens closer or farther from the eye. In practice, we refraction at a distance that matches where glasses sit when worn, which is about 12 to 15 millimeters for most adults. Refracting at this range makes the measured refractive error translate most accurately into a spectacle prescription that works well at the usual wear distance. If the distance is too close, like 8–10 mm, the lenses would need to be stronger to have the same effect at the eye, leading to a prescription that might feel too strong once made into glasses. If it’s too far, like 20 mm, the correction would be weaker than needed, leading to under-correction. So the best range is 12–15 mm.