Toric lens over-refraction
Order the next soft toric from its rotation and an over-refraction.
About this calculator
A soft toric lens that settles rotated puts its cylinder at the wrong axis, and the over-refraction then shows a crossed cylinder even when the lens powers are right. The order is the lens on the eye plus the over-refraction, with the over-refraction’s axis shifted by the amount the lens rotated.
The marker is read from the examiner’s side. A lens that rotates to the examiner’s left (clockwise) adds those degrees to the axis, and one that rotates to the right (counterclockwise) subtracts them. Two sphero-cylinders at different axes combine by adding their power vectors, which is why the axis cannot simply be added to the lens on the eye.
Formula
Order = lens + over-refraction with its axis shifted by the rotation (left adds, right subtracts), summed as power vectors
Worked example
A trial lens of plano with an over-refraction of −1.25 −1.00 × 100, and the lens marker 15° counterclockwise, orders −1.25 −1.00 × 085.
Sources
- Contact Lenses in Ophthalmic Practice, ch. 11, Astigmatism and Toric Contact Lenses
- Geometric, Physical and Visual Optics, ch. 13, §13.10–13.13
- Modern Ophthalmic Optics, ch. 5, §5.2.4 (power vectors)