Eyewear Dispensing and Frame Fitting
What this domain tests
Dispensing questions test whether you can translate a prescription and a patient's facial measurements into a frame that centers optics correctly, fits comfortably, and stays aligned. Expect boxing-system arithmetic, PD matching, vertex distance, and adjustment order.
Core rules
- In the boxing system, lens width A plus bridge DBL equals the frame PD. The geometric center of each lens is half the frame PD from the bridge centerline.
- For a symmetric frame, decentration per lens = frame PD/2 − monocular PD. Decenter inward when the frame PD is larger than the patient's PD.
- Increasing vertex distance strengthens a minus lens and weakens a plus lens at the eye; decreasing vertex distance does the reverse.
- Pantoscopic tilt lowers the optical center relative to the pupil and should be matched to the frame's intended tilt. Face form wraps the frame to the face.
- Fit order: bridge and nose pads first, then temple length and bend, then tilt and alignment. Comfort and optical centration must both be checked.
Worked example
A frame is marked 52-18, so its frame PD is 52 + 18 = 70 mm. The patient's binocular PD is 64 mm, or 32 mm per eye. Each lens must be decentered inward by 70/2 − 32 = 35 − 32 = 3 mm. If the patient instead has monocular PDs of 31 and 33 mm, the right lens decenters 4 mm and the left 2 mm; using a single 3 mm average would misplace both optical centers.
Traps
Confusing A with frame PD; using binocular PD for monocular decentration; ignoring vertex distance when a high-power lens is refitted; and adjusting temples before the bridge is stable. Always record monocular PDs and verify the fitted optical center.