If a patient has 0.5 degree of visual field loss, how many prism diopters would that correspond to?

• A. 0.5 pd
• B. 1 pd
• C. 2 pd
• D. 3 pd

Answer: (B)

The effect of a prism in expanding the visual field is about angular deflection produced by the prism. Clinically, 1 diopter shifts the image by roughly 1 cm at 1 meter, which corresponds to about 0.57 degrees of angular deviation. To match a field loss of 0.5 degrees, you’d need around 0.5 / 0.57 ≈ 0.9 diopters, which is practically rounded to 1 diopter. So the best match is 1 prism diopter.

Using less than that would give a too-small shift (for example, 0.5 diopter is roughly 0.28 degrees), while more would overshoot (2 diopters is about 1.1 degrees). This approximation is why 1 diopter is the commonly chosen value for a 0.5° field change.