Back vertex distance (BVD)

 Back vertex distance (BVD)

BVD is the distance between the back surface of a corrective lens and the front of the cornea. It is an important factor in determining the effective power of a lens.

The effective power of a lens is the power that the lens has when it is worn at the prescribed distance from the eye. The prescribed distance is typically 12 mm for glasses and 0 mm for contact lenses.

BVD can affect the effective power of a lens in two ways:

Magnification: When a lens is moved closer to the eye, it magnifies the image. This is because the lens is now closer to the nodal point of the eye.

Minification:When a lens is moved further away from the eye, it minifies the image. This is because the lens is now further away from the nodal point of the eye.

The following table shows how BVD affects the effective power of a plus and minus lens:

It is important to note that BVD is only one factor that affects the effective power of a lens. Other factors include the lens power, the lens form, and the pantoscopic tilt.

Why is BVD important?

BVD is important because it affects the accuracy of the prescription lens. If the BVD is not measured accurately, the prescription lens may not correct the patient's vision properly.

BVD is also important for patients who have high prescriptions. In these patients, a small change in BVD can make a big difference in the effective power of the lens.

How is BVD measured?

BVD  can be measured using a variety of methods, including:

Optical vertometer: An optical vertometer is a device that measures the power of lenses and frames. It can also be used to measure BVD.

Vertex distance ruler:A vertex distance ruler is a simple tool that measures the distance between the back surface of a lens and the front of the cornea.

Keratometer: A keratometer is a device that measures the curvature of the cornea. It can also be used to estimate BVD.

Conclusion

BVD is an important factor in determining the accuracy and effectiveness of a prescription lens. It is important for eye doctors to measure BVD accurately, especially for patients with high prescriptions.