For every prism diopter in a bifocal, you should adjust the seg height by:

• A. 1/2 mm
• B. 2/3 mm
• C. 1/3 mm
• D. 1 mm

Answer: (C)

When a vertical prism is added in a bifocal, the near portion’s image shifts relative to the wearer’s pupil. To keep the reading segment aligned with the eye, you adjust the seg height by a small amount. The practical rule is about 1/3 millimeter for each prism diopter. So, for every diopter of vertical prism, shift the segment position by roughly 0.33 mm in the direction that re-aligns the near segment with the pupil. For example, 2 prism diopters would require about 0.66 mm of seg height change. Note that the direction depends on the prism’s orientation, and exact amounts can vary with frame and individual fitting, but 1/3 mm per diopter is the standard guiding value.