Answers

• 4.1 a. The Jackson cross cylinder is a lens made of 2 cylinders of equal but opposite magnitude placed at 90° relative to each other; the spherical equivalent of the resulting lens is zero (see Fig 4-17). High-power Jackson cross cylinders are especially useful in refining the refraction in low vision patients.

• 4.2 c. Spherical aberration occurs in patients with large or dilated pupils. This aberration is caused when light rays are refracted as they travel through a widely dilated pupil and strike the peripheral crystalline lens. The periphery of the human lens is more curved than the center, so the incoming light rays show increased refraction compared with the light rays that strike the central lens. In retinoscopy, this can result in the appearance of different central and peripheral reflexes. Thus, it is always important to concentrate on the central light reflex when performing retinoscopy.

• 4.3 b. Figure 4-38 depicts a high-minus spectacle lens where there is minification of the image. Note the barrel-shaped distortion of the Amsler grid as viewed through the lens.

  Figure 4-38

  

  (Courtesy of Tommy Korn, MD.)

• 4.4 b. The far point of a +9.00 D lens is 111 mm (1/9 m) behind the lens. However, for the old lens to focus the image on the retina, it must be held by the frame 12 mm in front of the cornea. Thus, the far point of the patient’s hyperopia is located 99 mm (111 mm − 12 mm) behind the cornea. If the new frame is to be located 22 mm in front of the cornea, it should be placed 121 mm (99 mm + 22 mm) in front of the far point of the patient’s hyperopia. The power required for this new lens, therefore, is 1/0.121 m = +8.26 D. Because spectacle lenses come in 0.25 D steps, the answer is +8.25 D.

• 4.5 c. Figure 4-39 depicts an occupational multifocal lens known as a double D. This type of lens has the additive near power at the top and bottom of the spectacle lens; the distance power is in the middle. This design is especially useful for airline pilots, who must frequently look up at cockpit instrument panels that are in close proximity to one another and look down at printed flight material.

  Figure 4-39

  

  (Courtesy of Tommy Korn, MD.)

• 4.6 b. The Abbe number is a measure of chromatic aberration. The lower the Abbe number, the higher the amount of chromatic aberration present in the lens material. Spectacle lenses with a low Abbe number often require antireflective coating to minimize chromatic aberration that arises particularly when bright indoor light reflects off the lenses.

• 4.7 b. The far point of a −8.00 D lens is 125 mm (1/8 m) and is located in front of the lens. A patient’s myopic refractive error is corrected with a −8.00 D trial lens when the lens is placed 10 mm in front of the cornea (Fig 4-40). If the lens is moved to 14 mm in front of the cornea with her existing frame, the far point remains the same distance from the original −8.00 D lens. The location of the existing frame from the far point is 135 mm − 14 mm = 121 mm. The power of the lens must then be 1/0.121 m = −8.26 D. Because spectacle lenses come in 0.25 D increments, the answer is −8.25 D.

• 4.8 b. When a patient has anisometropia, which may arise after cataract surgery, for example, a large vertical prismatic effect is induced in the bifocal add. When the patient suddenly looks through the top of the bifocal segment, image jump occurs because of vertical prismatic effects through the spectacle lenses. Image jump is a problem because the human brain has a very limited capacity to fuse 2 images that are separated vertically, as in the case of a bifocal lens with anisometropia.

• 4.9 a. As the eyes look down to read through the add segment, there is an abrupt upward image jump at the top edge of the segment. This jump is due to the prismatic effect of the plus lens (the add segment). On the basis of the Prentice rule, the amount of jump depends on the power of the segment and on the distance from the top of the segment to the optical center of the segment.

  Figure 4-40

  

  (Illustration developed by Tommy Korn, MD.)

• 4.10 d. As a general rule, the number of prism diopters is approximately twice the angle in degrees. However, this equation works only for small angles (<20°). An angle of 45° means that at 1 m, a beam is deviated by 1 m (100 cm). Thus, 45° corresponds to 100.0∆. An angle of 90° corresponds to infinity in prism diopters.

• 4.11 c. In general, patients perceive image jump as a greater problem than image displacement. Flat-top segments minimize image jump because the optical center is near the top. In patients with myopia, flat-top segments also reduce prism displacement because the base-down effect of the distance portion is reduced by the base-up effect of the segment.

• 4.12 b. Aphakic spectacles will generate base-out prism with convergence when compared to contact lens correction. Adjustment of the optical centers, addition of base-in prism, or single-vision reading glasses may improve this problem.

• 4.13 c. In the chin-up position, the patient is effectively adding plus power to the distance prescription by viewing through the upper portion of the add. The patient has either undercorrected hyperopia or overcorrected myopia.

• 4.14 d. The induced prism is a result of the anisometropia of the distance correction. The bifocal add is equal, and there should be no induced prism contribution from that. If the top of the bifocal is 3 mm below the optical center of the lens and the patient is looking 4 mm below that, then the distance for the Prentice rule is 7 mm and the power difference is 3 D; therefore, 2.1∆ with the induced prism base in the more myopic correction (right side).

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.