If a patient has 4 degrees of visual field loss, approximately how many prism diopters would be prescribed?

• A. 6 pd
• B. 8 pd
• C. 10 pd
• D. 12 pd

Answer: (B)

Visual field loss is addressed by shifting the image with a prism, expanding the usable field. A practical rule is that 1 prism diopter shifts light by about 0.57 degrees of visual angle. To compensate for roughly 4 degrees of loss, you’d need about 4 ÷ 0.57 ≈ 7 prism diopters, so the closest standard prescription is 8 prism diopters. Thus, 8 pd best matches the amount of angular shift needed. Options like 6 pd would provide only ~3.4 degrees of shift (not enough), while 10 or 12 pd would over-shift to ~5.7 or ~6.8 degrees.