Seg Drop and Seg Inset for Multifocal Lenses

What Are Seg Drop and Seg Inset?

Seg drop and seg inset are two positioning measurements that determine exactly where the multifocal segment sits on a bifocal or trifocal lens relative to the distance optical center. Together, they ensure the near reading zone is in the correct vertical and horizontal location for comfortable, strain-free near vision.

These measurements work as a pair: seg drop positions the segment vertically, and seg inset positions it horizontally. Both must be accurate for the patient to look through the correct part of the lens when reading.

Seg Drop: Vertical Placement

Seg drop is the vertical distance from the distance optical center (OC) down to the top of the multifocal segment line. It tells you how far below the primary viewing point the near zone begins.

A larger seg drop places the reading segment lower on the lens. A smaller seg drop places it higher. The ideal seg drop positions the top of the segment at or near the lower eyelid margin, which is the standard fitting guideline for most flat-top bifocals.

Why Seg Drop Matters

• Too much seg drop (segment too low): The patient must excessively lower their gaze or tilt their head back to reach the reading zone, causing neck strain
• Too little seg drop (segment too high): The reading segment intrudes into the distance field, causing blur or image jump when looking straight ahead

Calculating Seg Drop

Seg drop is determined by the relationship between the OC height and the segment height:

Seg Drop = OC Height - Segment Height

For example, if the distance OC is at 24mm and the segment top is at 17mm (both measured from the bottom of the lens), the seg drop is 24 - 17 = 7mm.

Seg Inset: Horizontal Placement

Seg inset is the horizontal distance the near segment is shifted nasally (inward) from the distance optical center. This nasal shift compensates for the natural convergence of the eyes when focusing on near objects.

When you read, your eyes rotate inward to fixate on close material. If the near segment were directly below the distance OC without any inset, you would be looking through the temporal edge of the segment rather than through its center. Seg inset moves the segment nasally so your converged gaze passes through the correct part of the reading zone.

Standard Seg Inset Values

The typical seg inset for a standard reading distance of 40cm is approximately 2 to 2.5mm per eye. This value comes from the difference between the patient's distance PD and near PD divided by two:

Seg Inset per eye = (Distance PD - Near PD) / 2

For a patient with a distance PD of 64mm and near PD of 60mm: Inset = (64 - 60) / 2 = 2mm per eye.

Seg Drop and Inset in Different Lens Types

Relationship to Segment Height

Seg drop and segment height are related but different measurements:

• Segment height is measured from the bottom of the lens groove to the top of the segment. It is an absolute position on the lens.
• Seg drop is measured from the OC to the top of the segment. It is a relative distance between two lens features.

The lab uses both measurements together. Segment height tells them where to place the segment in the frame. Seg drop tells them the relationship between the distance OC and the near segment, which affects the patient's visual experience.

Practical Fitting

When fitting a bifocal patient:

1. Adjust the frame to its final wearing position
2. Mark the pupil center (this determines the OC position)
3. Mark or measure the lower eyelid position (this determines the segment height)
4. The distance between these two marks gives the seg drop
5. Calculate the seg inset based on the patient's distance and near PD difference
6. Communicate all measurements to the lab clearly

Key Takeaways

• Seg drop is the vertical distance from the distance OC down to the segment top
• Seg inset is the horizontal nasal shift of the segment from the distance OC
• Seg inset compensates for eye convergence during near viewing
• Standard seg inset is approximately 2-2.5mm per eye
• Seg drop differs from segment height: seg drop is relative (OC to segment), segment height is absolute (frame bottom to segment)
• Both measurements must be accurate for comfortable multifocal wear