Munawar Raj 🩺

 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 len...