American Board of Ophthalmology (ABO) Practice Exam

What is the typical power of a lens with a curvature that results in a -2.75 - 0.50 X 65 reading?

1. -1.75 - 0.50 X 65

2. -2.50 - 0.50 X 65

3. -2.75 - 0.75 X 65

4. -2.75 - 0.50 X 90

The correct answer is: -2.50 - 0.50 X 65

In assessing the typical power of a lens given a specific reading such as -2.75 - 0.50 X 65, the primary focus is on identifying the spherical component, which represents the overall power of the lens. The first component of the prescription, -2.75, indicates that it is a myopic (nearsighted) lens with a significant cylindrical correction of -0.50, oriented at an axis of 65 degrees. To determine which of the choices represents a typical power that aligns closely with the original reading, you analyze the spherical component first. The spherical power generally encapsulates the primary correction required for the patient's vision, while the cylindrical component addresses astigmatism. In this case, the sphere of -2.75 is consistent with option B, which also maintains a similar cylinder of -0.50. Other choices deviate either in the sphere's power or the axis, suggesting they would not accurately reflect the necessary correction as defined by the original prescription. Thus, option B correctly retains the spherical power component and maintains the cylindrical component's essential qualities, making it representative of the typical lens power for a reading of -2.75 - 0.50 X 65.