Patient Measurements & Fittings
PD, seg heights, vertex distance, frame fitting, and adjustments.
Pupillary Distance (PD)
PD — Distance between the centers of the pupils (mm).
Types:
- Binocular PD — Total distance (e.g., 64mm)
- Monocular PD — Distance from nose bridge to each pupil (more accurate)
Distance PD vs. Near PD:
- Near PD is approximately 3mm less than distance PD (convergence)
Why PD Matters:
- Optical center must align with the pupil
- Incorrect PD causes unwanted prismatic effect (Prentice's Rule)
Segment Height and Multifocal Measurements
Seg Height — Vertical distance from the lowest point of the lens to the top of the bifocal segment or fitting cross.
Standard Placement:
- Bifocal: Lower lid margin
- Progressive: Center of the pupil (fitting cross)
Pantoscopic Tilt Rule: For every 2 degrees of tilt, add 1mm to seg height.
Common Seg Types:
- FT-28: Most common bifocal (28mm wide)
- FT-35: Wider near field (35mm)
- Executive: Full-width segment
Vertex Distance and Other Measurements
Vertex Distance — Back of lens to cornea. Standard: 12-14mm.
Compensation Formula: Dc = D / (1 - d × D)
Key Rules:
- Moving minus lens closer → less minus needed
- Moving plus lens closer → more plus needed
Pantoscopic Tilt: For every 2° of tilt, add 1mm to progressive seg height.