Calculator
Vertex distance calculator
Find the power to order when a lens moves closer to or farther from the eye, such as from a phoropter to glasses or glasses to contact lenses.
Prescription and distances
Use 0 mm for a contact lens. A smaller “worn at” distance means the lens moves in, toward the eye.
Compensated Rx
The lens moves 12 mm in, toward the eye.
Exam method−7.25 −1.50 × 180
Exact optics−7.25 −1.75 × 180
The two methods round to different powers. Exam answers follow the workbook method.
| Meridian | DC | Effective (exam / exact) | Compensated (exam / exact) |
|---|---|---|---|
| −8.00 @180 | 0.77 | −8.77 / −8.85 | −7.23 / −7.30 |
| −10.00 @90 | 1.20 | −11.20 / −11.36 | −8.80 / −8.93 |
Exam: DC = mm × F² ÷ 1000; moving in, CP = F + DC. Exact: F ÷ (1 − d × F). Each meridian is rounded to 0.25 D and the Rx rebuilt in minus cylinder.