VD in eye tests usually refers to Vertex Distance, which is a crucial measurement when prescribing eyeglasses or contact lenses.
Understanding Vertex Distance
Vertex distance is defined as:
- The distance between the back surface of a corrective lens (glasses or contact lenses) and the front of the cornea (the clear front surface of the eye).
Why Vertex Distance Matters
Accurate vertex distance is particularly important for higher prescriptions. Here's why:
- Lens Power: The effective power of a lens changes depending on its distance from the eye.
- Prescription Accuracy: If the vertex distance used during the eye exam is significantly different from the distance at which the glasses are worn, the effective power of the glasses may be incorrect, leading to blurred vision or discomfort.
How Vertex Distance is Measured
During an eye exam:
- The optometrist or ophthalmologist measures the vertex distance using a device.
- This measurement is then used to adjust the lens power in the eyeglass prescription to compensate for the distance.
Vertex Distance in Contact Lenses
- Contact lenses sit directly on the cornea, so the vertex distance is essentially zero.
- Due to this, if you switch from glasses to contacts, your prescription will need to be adjusted to account for the difference in vertex distance.
