Multifocal Segment and Fitting Height: How Opticians Measure It

Adjust the chosen frame to its wearing position first. For a lined bifocal, measure from the lowest point of the lens opening up to the lower lid margin. For a progressive, measure to the center of each pupil separately and record it with that eye’s monocular PD. Minimum fitting heights depend on the lens design.

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Adjust before measuring

Heights only mean something in the frame being ordered. Take them in that frame, or one of the same size and style, after it has been adjusted to sit as it will be worn. The horizontal centration has to be right, too, because the segment or fitting cross is placed relative to both.

Lined bifocals and trifocals

1. Sit with your eyes level with the patient’s. Ask them to fix on the bridge of your nose.
2. Hold a millimeter rule upright in front of the eye, zero at the chosen landmark. For a bifocal that is the lower lid margin, or the lower limbus if your practice uses it.
3. Read down to the lowest point of the lens opening (in a full-rim frame, the bottom of the groove inside the eyewire). On a slanted shape, that may not be straight below the pupil.
4. Repeat for the other eye.

Segment height runs from that lowest point up to the top of the segment. Segment drop is defined against a different reference, the horizontal centerline of the lens shape, so check which one an order form asks for.

Trifocals use a different landmark. The top of the intermediate segment goes at the lower edge of the pupil, not the lower lid. Zero the rule at the lower pupil margin, with the patient looking straight ahead, and read down to the lowest point of the lens opening in the same way. Treat that number as a starting point: the patient’s previous glasses and how they use them can move it.

Progressives

A progressive is placed by its fitting cross, usually at the center of the pupil. Mark each pupil center on the lens or on clear tape, with the patient looking straight ahead. Read each eye’s height separately, between the mark and the lowest point of the opening. The design’s fitting guide sets its minimum height.

Monocular PD tie-in

Each fitting cross must sit in front of its own pupil, so a progressive needs a monocular PD for each eye as well as a height. The decentration calculator turns frame size and monocular PDs into how far each optical center moves in.

Checks

• Heights for the two eyes are measured separately, never copied across.
• The frame adjustment is unchanged since you measured.

Practice set

Original problems.

1. The lower lid margin sits 19 mm above the lowest point of the right lens opening. Where does the bifocal top go?

   Show answer

   19 mm segment height, measured from the lowest point of the opening.

2. PAL fitting: right pupil center 23 mm above the lowest point of the opening, left 22 mm. What do you record?

   Show answer

   R 23 mm, L 22 mm. Record each eye separately; do not copy one height to both.

3. Frame A 54 mm, DBL 16 mm. Monocular PDs R 32.0, L 31.0. How far in does each optical center move from the box center?

   Show answer

   Frame PD 54 + 16 = 70, half is 35. R 35 − 32 = 3.0 mm in; L 35 − 31 = 4.0 mm in.

4. You measured heights, then the patient asks for the nose pads to be lowered. What now?

   Show answer

   Remeasure. Moving the frame moves every height.

5. A patient’s frame leaves 13 mm below the pupil. Is that enough for a progressive?

   Show answer

   It depends on the design. Check that lens’s fitting guide before ordering.

Standard text used for this page: Brooks CW, Borish IM. System for Ophthalmic Dispensing, 3rd ed. Butterworth-Heinemann, ch. 5 (bifocal height methods) and ch. 20 (progressive fitting cross height and troubleshooting). ABDO’s 2024 measurement review is linked under sources.

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