American Board of Ophthalmology (ABO) Practice Exam

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A -6.50 lens decentered 2 mm results in how much prism?

The correct answer is: 1.3

To determine how much prism is induced when a lens is decentered, you can use the formula for prism diopters, which is calculated as the power of the lens (in diopters) multiplied by the distance the lens is decentered (in centimeters). In this case, the lens has a power of -6.50 diopters. When the lens is decentered by 2 mm, we first convert that measurement to centimeters, which means 2 mm equals 0.2 cm. The formula for calculating prism is: Prism (in prism diopters) = Lens power (in diopters) × Decentration distance (in cm) By applying the values: Prism = -6.50 diopters × 0.2 cm = -1.3 prism diopters When considering prism in practical terms, we take the absolute value, so it becomes 1.3 prism diopters. This means that the decentered -6.50 lens when forced out of the optically centered position creates a prismatic effect of 1.3 prism diopters. This calculation highlights how the power of the lens and the amount of decentration directly correlate to the amount of prism created. Therefore, the calculated