Where should the bifocal line be on glasses?

The standard guideline is for the top of the bifocal segment line to align with the lower lid margin (the bottom edge of the lower eyelid) when the patient looks straight ahead in natural head position. This allows comfortable access to the near zone when looking down to read while keeping the segment out of the distance field of view.

How do you measure seg height?

With the properly adjusted frame on the patient's face, stand at the same eye level and mark the lower lid position (for bifocals) or lower pupil margin (for trifocals) on the demo lens. Remove the frame and measure the vertical distance from the lowest point of the lens opening to the mark. Record in millimeters.

What happens if seg height is wrong?

If the seg is too high, the patient sees the segment line in their distance vision, causing image jump and the urge to tilt the head back. If the seg is too low, the patient must drop their gaze excessively to reach the reading zone, causing neck strain and discomfort. Either error typically requires a remake.

Is seg height the same for trifocals as bifocals?

No. Trifocal seg height is typically higher than bifocal seg height. The trifocal segment top is placed at the lower pupil margin (rather than the lower lid margin for bifocals) because the intermediate zone needs to be readily accessible without excessive downward gaze.

Segment Height Measurement: Positioning Bifocal and Trifocal Segments

What Is Segment Height?

Segment height (often abbreviated "seg height") is the vertical distance from the lowest point of the lens opening in the frame to the top of the bifocal or trifocal segment line. This measurement determines where the near-vision segment is positioned vertically in the finished lens, directly affecting the patient's ability to access the near zone comfortably.

Standard Placement Guidelines

The traditional guideline for bifocal segment placement:

Bifocal seg height: The top of the segment line should align with the lower lid margin of the patient's eye when they look straight ahead in natural head position
Trifocal seg height: The top of the trifocal segment should align with the lower pupil margin, slightly higher than a bifocal, because the intermediate zone needs to be accessible without excessive downward gaze

Lens Type	Seg Top Position	Reference Point
Bifocal (standard)	Lower lid margin	Natural gaze, straight ahead
Trifocal	Lower pupil margin	Natural gaze, straight ahead

These are starting guidelines. The actual seg height should be adjusted based on the patient's occupational needs, reading habits, and personal preference. A patient who reads extensively may prefer a higher seg for easier access. A patient who drives primarily may prefer a lower seg to minimize the visible segment in their distance field.

How to Measure Segment Height

Have the patient wear the selected frame, properly adjusted, in their natural head position
Stand directly in front of the patient at the same height
Using a ruler or frame marking pen, mark the position of the lower lid margin (for bifocals) or lower pupil margin (for trifocals) on the demo lens or frame
Remove the frame and measure the vertical distance from the lowest point of the lens opening (in the frame) to the mark
Record the measurement in millimeters

Always measure seg height with the frame properly adjusted on the patient's face. If the frame is sitting too high or too low during measurement, the seg will be in the wrong position when the finished glasses are dispensed. Adjust the frame first, then measure.

Common Seg Height Errors

Seg Too High

The patient sees the segment line in their distance vision
Objects in the distance may appear to "jump" at the segment boundary
The patient may tilt their head back to look through the distance zone

Seg Too Low

The patient must drop their gaze excessively to reach the near zone
Reading is uncomfortable because the eyes must look far down
The patient may tilt their head forward, causing neck strain

Measuring seg height while the patient is looking up at you (because you are standing while they are seated). This positions the eyes too high in the frame, resulting in a seg height that is too high. Always measure at the same eye level as the patient, ideally both sitting or both standing.

Seg Height for Different Frame Types

Full-rim frames: Measure from the deepest point of the lower rim
Semi-rimless frames: Measure from the lowest point of the lens opening (which may be difficult to define precisely; use the demo lens as a guide)
Rimless frames: Measure from the lowest point of the demo lens or pattern lens

Monocular Seg Heights

Just as monocular PD is more accurate than binocular PD, monocular seg heights (separate measurements for each eye) account for facial asymmetry. One eye may sit slightly higher in the frame than the other. For most bifocal fittings, a single seg height is adequate, but for patients with noticeable facial asymmetry, monocular seg heights improve comfort.

Clinical Relevance

Seg height is one of the measurements most directly felt by the patient. An incorrect seg height is immediately noticeable: the patient either sees the line in their distance vision or cannot find the reading zone. Getting the seg height right on the first dispensing prevents remakes and establishes credibility as a skilled optician.

Key Takeaways

Seg height is measured from the lowest point of the lens opening to the top of the segment line
Standard bifocal placement: segment top at the lower lid margin
Standard trifocal placement: segment top at the lower pupil margin
Always measure with the frame properly adjusted on the patient's face
Measure at the same eye level as the patient to avoid parallax
Adjust the standard placement based on the patient's specific needs and preferences