What decentration value is required per lens when the frame PD is 60 mm and the patient PD is 58 mm?

What decentration value is required per lens when the frame PD is 60 mm and the patient PD is 58 mm?

• A. 0 mm
• B. 1 mm
• C. 2 mm
• D. 3 mm

Answer: (B)

When deciding how much to decenter each lens, use half of the difference between the frame Pupillary Distance (PD) and the patient’s PD. This keeps the optical centers aligned with the eyes while maintaining the frame’s overall spacing. Here, frame PD is 60 mm and patient PD is 58 mm, a difference of 2 mm. Half of that is 1 mm, so each lens should be decentered by 1 mm. If you didn’t decenter at all, there would be a 2 mm misalignment between frame centers and pupil centers. Decentering 2 mm per lens would overcorrect by moving more than the needed amount, and 3 mm would be even further off. Therefore, the correct decentration per lens is 1 mm.

When deciding how much to decenter each lens, use half of the difference between the frame Pupillary Distance (PD) and the patient’s PD. This keeps the optical centers aligned with the eyes while maintaining the frame’s overall spacing.

Here, frame PD is 60 mm and patient PD is 58 mm, a difference of 2 mm. Half of that is 1 mm, so each lens should be decentered by 1 mm.

If you didn’t decenter at all, there would be a 2 mm misalignment between frame centers and pupil centers. Decentering 2 mm per lens would overcorrect by moving more than the needed amount, and 3 mm would be even further off. Therefore, the correct decentration per lens is 1 mm.