The Importance of Vertex Distance
For any spherical correcting lens, the distance from the lens to its focal point is constant. Changing the position of the correcting lens relative to the eye also changes the relationship between the F2 of the correcting lens and the far point plane of the eye. With high-power lenses, as used in the spectacle correction of aphakia or high myopia, a small change in the placement of the lens produces considerable blurring of vision unless the lens power is altered to compensate for the new lens position so that the secondary focal point of the lens coincides with the far point of the eye.
Figure 4-20 A diverging lens is used to correct myopia.
Figure 4-21 A converging lens is used to correct hyperopia.
With refractive errors greater than ±5.00 D, the vertex distance must be accounted for in prescribing the power of the spectacle lens. A distometer (also called a vertexometer) is used to measure the distance from the back surface of the spectacle lens to the cornea with the eyelid closed (Fig 4-22A). Moving a correcting lens closer to the eye—whether the lens has plus or minus power—reduces its effective focusing power (the image moves posteriorly away from the fovea), whereas moving it farther from the eye increases its focusing power (the image moves anteriorly away from the fovea).
For example, in Figure 4-23 the +10.00 D lens placed 10 mm in front of the cornea provides sharp retinal imagery. Because the focal point of the correcting lens is identical to the far point plane of the eye and because this lens is placed 10 mm in front of the eye, the far point plane of the eye must be 90 mm behind the cornea. If the correcting lens is moved to a new position 20 mm in front of the eye and the far point plane of the eye is 90 mm behind the cornea, then the focal length of the new lens must be 110 mm, requiring a +9.10 D lens for correction. A contact lens will need to have a power of 11.10 D. This example demonstrates the significance of vertex distance in spectacle correction of large refractive errors. Thus, the prescription must indicate not only the lens power but also the vertex distance at which the refraction was performed. The optician must recalculate the lens power as necessary for the actual vertex distance of the chosen spectacle–frame combination. Clinical Example 4-3 demonstrates examples of vertex distance. See also Chapter 1, Example 1-6.
Figure 4-22 Measuring vertex distance. A, Vertexometer (distometer). B, Corneal alignment tool.
(Part A courtesy of Tommy Korn, MD; part B courtesy Thomas F. Mauger, MD.)
Figure 4-23 The importance of vertex distance in the correction of high refractive errors.
Allowing for vertex distance with the phoropter is made easier due to 2 commonly found features. The first feature is that the lenses within the phoropter, even though found in different planes, have their power referenced to the back surface of the most posterior lens in the system. The second feature is that if the cornea of the eye is aligned at the zero mark on the corneal alignment tool (Fig 4-22B), the vertex distance equals that assumed for the phoropter (13.75 mm for the Reichert Phoroptor) and has an accompanying table with the proper conversion for each mm anterior or posterior that the cornea is positioned. The patient should be refracted in a trial frame or overrefracted in their current spectacles if the vertex distance varies significantly (>6 mm) from that assumed in the phoropter.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.