Certified Paraoptometric (CPO) Practice Exam 2025

Why might a contact lens prescription differ from a spectacle prescription?

1. Because of the lens material used

2. Of the vertex distance between the contact lens and the spectacle lens

3. Due to the type of vision correction required

4. Because of the color of the lenses

The correct answer is: Of the vertex distance between the contact lens and the spectacle lens

The reason a contact lens prescription may differ from a spectacle prescription primarily revolves around the vertex distance between the two types of lenses. Vertex distance refers to the distance from the back surface of the spectacle lens to the front surface of the cornea. Since contact lenses sit directly on the eye, they are much closer to the cornea than spectacles, which can significantly affect the power of the lens needed to achieve the same vision correction. When light passes through a lens, the distance from the lens to the eye changes how the light is bent. For individuals with stronger prescriptions, this difference can lead to a notable variance in the power required for the contact lenses in comparison to glasses. Consequently, adjustments must be made in the contact lens prescription to ensure optimal vision correction. Other factors, such as lens material and color, can impact comfort or aesthetics but do not directly factor into the basic prescription differences. Likewise, while the type of vision correction needed can lead to different lenses, it does not explain the intrinsic differences in measurement and adjustment needed between contact lenses and spectacles.