A patient with an Rx of +2.50 + 0.50 x 180 fit .50 flatter than K would need a contact with what power?

To determine the appropriate contact lens power for a patient with a prescription of +2.50 +0.50 x 180, it is essential to consider the concept of fitting contact lenses in relation to the patient's corneal curvature, which is represented by the K values (Keratometry readings). The patient is to be fitted with a contact lens that is 0.50 diopters flatter than the K value.

The patient's prescription indicates that they have a spherical component of +2.50 diopters and a cylindrical component of +0.50 diopters at an axis of 180 degrees. To calculate the contact lens power, the following steps are generally followed:

1. Determine the vertexed power: Since the contact lens is placed directly on the eye, there is no need for vertex distance compensation, which is typically required for glasses.

2. Adjust for corneal shape: Fitting the contact lens flatter than the K value involves assessing the spherical equivalent of the prescription. In this case, the spherical equivalent can be calculated as:

• Spherical equivalent = Sphere + (Cylinder/2)

• From the prescription: +2.50 + (0.50/2) = +2.50 + +0