Figure 3-22 A vertexometer (distometer) is used to measure the vertex distance from the back surface of the spectacle lens to the cornea through a closed eyelid. (Courtesy of Tommy Korn, MD.)

For example, in Figure 3-23 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, then the focal length of the new lens must be 110 mm, requiring a +9.10 D lens for correction. 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.

Figure 3-23 The importance of vertex distance in the correction of high refractive errors.

Cylindrical Correcting Lenses and the Far Point Concept

The far point principles used in the correction of hyperopia and myopia are also employed in the correction of astigmatism with spectacle lenses. However, in astigmatism, the required lens power must be determined separately for each of the 2 principal meridians.

Cylinders in spectacle lenses produce both monocular and binocular distortion. The primary cause is meridional aniseikonia—that is, unequal magnification of retinal images in the various meridians. Although aniseikonia may be corrected by iseikonic spectacles, such corrections may be complicated and expensive, and most practitioners prefer to prescribe cylinders according to their clinical judgment. Clinical experience also suggests that adult patients vary in their ability to tolerate