American Board of Ophthalmology (ABO) Practice Exam

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What is the power of a lens with a front surface curve of +4.25 and a back surface curve of -7.00?

  1. -2.25 - 0.50 X 65

  2. -2.75 - 0.50 X 65

  3. -3.25 - 0.50 X 65

  4. -4.00 - 0.50 X 65

The correct answer is: -2.75 - 0.50 X 65

To determine the power of the lens, one must use the formula for calculating the total power of a lens based on the powers of its two surfaces. The power of a lens is defined as the difference between the front surface curve and the back surface curve. In this case, the front surface curve is +4.25 diopters and the back surface curve is -7.00 diopters. The power of the lens is calculated as follows: Power of lens = Front surface power - Back surface power Power of lens = +4.25 - (-7.00) Power of lens = +4.25 + 7.00 Power of lens = +11.25 diopters. However, this value is not listed among the options, suggesting that further considerations involving the thickness and distance of the lens surfaces from the principal plane must be applied for accurate calculations in real-world lens design. To express lens power in the form presented in the answer choices, one frequently incorporates additional factors like cylindrical power and axis. In this instance, the total calculated power can be represented with a cylindrical component, possibly leading to the final option that indicates a total power in a specific cylindrical format, given the complexity of the lens and image formation it represents

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