can you convert a glasses prescription to a contact lens prescription?

This query is quite common with people. It is difficult to answer this with a simple yes or no. To understand the possibility of converting a glass prescription into a contact lens prescription, it is important to understand the difference between the two.

For the uninitiated, both glasses and contact lens correct refractive errors of the eye like:

  • Short-sightedness or myopia,
  • Long-sightedness or hyperopia,
  • Astigmatism which results in distorted or blurred vision at all distances and
  • Presbyopia wherein the eyes are unable to focus on objects situated close by.

Difference between a glass and contact lens prescription

An optometrist’s prescription for the glasses and contact lenses might look similar to a layman, but there are certain distinct differences. This arises because:

  • Glasses are worn on the bridge of the nose, some distance away from the eyes while the contact lens sits directly over the pupil and
  • An axis and cylindrical value are also attached to glasses but absent in contact lens, however, these measurements are present in toric and multifocal contact lens.

Thus the two, glasses and contact lens, require two different sets of tests and eye power measurements. The type of contact lens required and its strength depend on the present refractive error and the respective correction required. Thus glass prescriptions cannot be used to avail contact lens and vice-versa.

It is very important to understand that using Johnson and Johnson lenses with the wrong power to address the refractive error often results in headaches, blurry vision, double vision, eye strain etc.

Converting glass power to contact lens power

There are, however, 3 different ways by which the power prescribed in a glass prescription can be converted into that of the contact lenses. They are:

  • Visiting an ophthalmologist: This is the most common method. An ophthalmologist or an eye doctor is trained to:
    • Access the refractive error present in the eye and
    • Prescribe glasses and contact lens as required by the patient.

Herein the eye doctor, with the help of the optometrist, provides two different prescriptions for people who want to use a combination of glasses and contact lens. This is by far the safest and best way to obtain a contact lens prescription.

  • Using Black Vertex formula: This mathematical equation is popularly used by eye doctors and optometrists to theoretically convert a glass power into a contact lens power. The general formula states that:

Fc = F/(1 – xF) wherein,

Fc is the contact lenses theoretical power

F the power present in the glasses

X the vertex distance

This is a slightly complicated method which requires special training to understand and implement.

  • Using the Black Vertex charts: This helps make life easier for the ophthalmologists and optometrists. This is a reference chart which lists the vertex conversion values for glasses and contact lens. These values are pre-calculated using the Black vertex formula given above. Thus this chart helps the eye clinic professionals to save time yet make accurate power predictions for both the glasses and contact lens.

While there are ways to convert the power of glasses into Johnson and Johnson lenses, a layman can only order either of the two based on a valid prescription obtained from the optician. It is better not to take risks since the eye is vulnerable and prone to permanent damage.