Eyeglass Prescription - Variations in Prescription Writing

Variations in Prescription Writing

There is a surprising amount of variation in the way prescriptions are written; the layout and terminology used is not uniform.

When no correction is needed, the spherical power will sometimes be written as 0.00 and sometimes as plano (pl.). The lens, although not flat, is optically equivalent to a flat piece of glass, and has no refractive power.

When cylindrical correction is needed, the mathematics used to denote the combination of spherical and cylindrical power in a lens can be notated two different ways to indicate the same correction. One is called the plus-cylinder notation (or "plus cyl") and the other the minus-cylinder notation (or "minus cyl"), based upon whether the axis chosen makes the cylindrical correction a positive or negative number. The method to transform one format to another is called flat transposition.

For example, these two prescriptions are equivalent:

The plus-cylinder notation shows the prescription as a correction of +2.00 diopters along an axis of 150° and an additional correction of +1.00 diopters, giving a total correction of (+2.00) + (+1.00) = +3.00 diopters, at 90 degrees from that meridian (= 60°).

The minus-cylinder notation shows the prescription as a correction of +3.00 diopters along an axis of 60° and an additional correction of −1.00 diopters, giving a total correction of (+3.00) + (−1.00) = +2.00 diopters, at 90 degrees from that meridian (= 150°).

The result in both cases is +2.00 diopters at the 150th meridian and +3.00 diopters at the 60th meridian.

In practice, optometrists tend to use minus-cylinder notation, whereas ophthalmologists and orthoptists tend to prescribe using plus-cylinder notation. However, some ophthalmologists and orthoptists (such as in Australia) are changing to using minus-cylinder notation.

In addition to the plus and minus cylinder notations, some countries use slight variations for special purposes. For example, the National Health Service of the United Kingdom uses the term Greatest Spherical Power when looking up the amount of state optical benefits that can apply to a particular prescription. This is simply the transposition of the prescription format so that the magnitude of the sphere is greatest. In the examples given earlier this would be the minus-cylinder version; that is, +3.00 −1.00 x 60° as opposed to +2.00 +1.00 x 150°.