Approximately how many prism diopters correspond to one degree of visual field when prescribing prism for patients with visual field defects?

• A. 1 pd
• B. 2 pd
• C. 3 pd
• D. 4 pd

Answer: (B)

Prism power relates to how much the light is angularly deflected. A 1 diopter prism deflects a light ray enough that, at a distance of 1 meter, the lateral shift is about 1 centimeter, which corresponds to an angular deviation of arctan(0.01) ≈ 0.57 degrees. So each prism diopter gives roughly 0.57 degrees of shift. To achieve a 1-degree shift, you’d need about 1 / 0.57 ≈ 1.75 diopters, which is about 2 diopters. Therefore, approximately two prism diopters correspond to one degree of visual field change when prescribing prism for field defects, keeping in mind this is a distance-dependent rule of thumb.