Answers

• 3.1. b. Utilizing the concepts of the nodal point and similar triangles, retinal image size is related to Snellen letter height (18 mm), the distance from the eye to the eye chart (6 m), and the distance from the nodal point to the retina (17 mm), as follows:

  

  The retinal image height of a 20/20 letter is half of this, about 0.025 mm.

• 3.2. d. Snellen letters are constructed such that they subtend an angle of 5 arcmin when located at the distance specified by the denominator (here, 40 ft). At a distance of 20 ft, the angle subtended by a 20/40 Snellen letter is twice that at 40 ft, or 10 arcmin.

• 3.3. d. The concept of the nodal point is applicable for the Goldmann perimeter, in which the target lights are located at 33 cm (330 mm) from the cornea. Because this distance is approximately 20 times greater than the 17-mm distance from the nodal point to the retina, the target lights in the perimeter are approximately 20 times larger than their corresponding retinal images.

• 3.4. The far point of an emmetropic eye is located at optical infinity. For a myopic eye, the far point is located at a finite distance in front of the cornea. For a hyperopic eye, the far point is located behind the retina.

• 3.5. c. One focal line is in front of the retina, the other is behind the retina. The circle of least confusion may be located very close to the retina (see Chapter 1), allowing such patients to see reasonably well even without optical correction if the magnitude of the astigmatism is not great.

• 3.6. d. The patient’s amplitude of accommodation is 6.00 D, because it takes 1.00 D of accommodation to focus to infinity and an additional 5.00 D of accommodation to focus to the false near point at 20 cm.

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.