Question
A patient selects a small, round frame and has a prescription of -14.00 D Sphere. The frame allows for a minimum vertex distance of 9 mm. The default lab layout would normally place the vertex distance at 14 mm. 1. Calculate the required prescription power change if you adjust the vertex distance of the finished frame down from 14 mm to 9 mm. 2. Analyze the optical advantages of reducing the vertex distance for a high-minus patient regarding field of view and retinal image magnification.
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• Part 1: Vertex Power Calculation.
• Target plane is moving 5 mm closer to the eye (d = -0.005 m).
• Formula: F_new = (F)/(1 - dF) = (-14.00)/(1 - (-0.005 × -14.00)) = (-14.00)/(1 - 0.07) = (-14.00)/(0.93) ≈ -15.05 D.
• The ordered power must be increased to -15.00 D to maintain the same effective correction at the closer vertex distance.
• Part 2: Optical Advantages. * Bringing the lens closer to the eye increases the patient's field of view through the clear optical zone.
• It also reduces the severe minification associated with high-minus lenses, making the retinal image larger and improving overall visual acuity and contrast perception.
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