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

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.

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.