Understanding Your Eyeglass Prescription

In most cases, the correction is not the same for both eyes. Each eye has its own prescription line.

The correction formula consists of 1 to 3 elements and designates the corrective power of the lens. These elements are numerical values ​​that correspond to the power of the lens expressed in diopters, which is the unit of measurement used.

1. Spherical Power (SPH)

The spherical power appears first on the prescription. If the value is preceded by a “ +” , the lens is said to be convergent and allows the correction of hyperopia (near vision problem) , if the value is preceded by a “-” , the lens is said to be divergent and allows the correction of myopia (distance vision problem). The mention is preceded by “ OD ” for the right eye and “ OG ” for the left eye. 

2. Cylindrical Power (CYL)

Cylindrical power is used to correct astigmatism . It appears second on the prescription and is indicated in parentheses.

3. Axis of Rotation Relative to the Horizontal

This is the third value on the prescription, it is only present to complement the second. The axis of rotation is crucial for astigmatism. It indicates the direction of the cylindrical correction necessary to obtain clear vision.

4. Correction for Presbyopia

To correct presbyopia (progressive loss of the eye's ability to focus on near objects) Progressive lenses are generally used. On the prescription, this will be the fourth numerical value. It corresponds to the spherical addition. This value is always preceded by the term “Addition” (or Add) and the “ + ” sign. In the vast majority of cases, the value is the same for both eyes.

5. Examples

- Example 1:

- Prescription: OD: +2.00 (-1.25) 55°

OG: +1.75 (-1.25) 65°

- Interpretation: The wearer is hyperopic and astigmatic in both eyes. On the right, he is hyperopic by 2 diopters and he is astigmatic by 1.25 diopters and the lens must be oriented at 55 degrees from the horizontal. On the left, he is hyperopic by 1.75 diopters and he is astigmatic by 1.25 diopters and the lens must be oriented at 65 degrees from the horizontal.

- Example 2:

- Prescription: OD: -4.50 (-1.25) 10° Add 2.00

OG: -2.75 (-1.00) 160° Add 2.00

- Interpretation: The wearer is myopic , astigmatic and presbyopic in both eyes. On the right he is myopic by 4.50 diopters, astigmatic by 1.25 diopters and the lens is oriented at 10° from the horizontal. On the left he is myopic by 2.75 diopters, astigmatic by 1 diopter and the lens is oriented at 160° from the horizontal. Presbyopia requires a spherical addition of 2 diopters for both eyes.

Conclusion

Each prescription for eyeglass lenses is unique and precise, taking into account the spherical power, the cylindrical power with its axis, and possibly an addition for presbyopia. These parameters are essential to guarantee optimal visual correction adapted to each individual. It is recommended to regularly consult an eye care professional for adjustments.