Certified Ophthalmic Technician (COT) Practice Test 2025 - Free COT Practice Questions and Study Guide

To determine the spectacle lens power required to correct a patient's refractive error to emmetropia, it's essential to understand the concept of far point and how it relates to visual acuity. The far point is the maximum distance at which an object can be seen clearly without accommodation. In this case, the patient's far point is 20 cm (or 7.87 inches), indicating that they can only see clearly at this close distance, meaning they are likely myopic (nearsighted).

To achieve emmetropia (normal vision where objects can be seen clearly at a distance), the far point should be at infinity. To calculate the required lens power, we can use the formula for lens power (P), which is the inverse of the focal length (f) in meters:

P (in diopters) = 1 / f (in meters).

Since the patient's far point is at 20 cm, we first convert this distance into meters, which is 0.2 meters. The lens needed should effectively bring the far point to infinity.

Thus, the calculation is:

P = 1 / (-0.2) = -5.00 D.

The negative sign indicates that a diverging lens is needed, which is typically