Frame Fitting Fundamentals
Proper spectacle fitting is as important as an accurate prescription -- a poorly fit frame can impair vision even with a correct Rx. The CPOA often assists with or performs frame measurements, pupillary distance (PD) measurements, and basic frame adjustments. Understanding fitting principles ensures patient comfort, optimal optical performance, and lens durability.
Frame Measurements
Spectacle frames use a standardized measurement system called the boxing system. Key dimensions:
| Measurement | Description |
|---|---|
| Eye size (A) | Horizontal width of the lens opening (box) |
| Bridge size (DBL) | Distance between the two lenses (distance between lenses) |
| Temple length (B-length) | Length of the arm from hinge to the ear bend |
| Vertical depth (B) | Vertical height of the lens opening |
Frame size is typically expressed as Eye size / Bridge size (e.g., 52/18). A longer temple or narrower bridge may be needed for different facial anatomies.
Pupillary Distance (PD)
Pupillary distance (PD) is the distance between the pupil centers, measured in millimeters. It determines where to center the optical portion of each lens in the frame so the patient looks through the optical center of their lens.
Types of PD Measurement
- Binocular PD: total distance from pupil center to pupil center (e.g., 64 mm)
- Monocular PD: distance from each pupil center to the center of the nose bridge (e.g., OD: 32 / OS: 32). Monocular PDs are preferred because most people are slightly asymmetric.
How to Measure PD
Using a millimeter ruler or pupilometer:
- Seat the patient at eye level, facing you
- Close your right eye; patient fixates on your left eye
- Align the zero mark on the ruler with the center of the patient's right pupil
- Open your right eye; close your left eye -- the ruler value at the patient's left pupil is the binocular PD
- For monocular PD, note the distance from the nose bridge to each pupil
💡 Clinical Tip: A pupilometer automates PD measurement and is more precise than a ruler for patients with strabismus or poor fixation. Always measure for distance and near separately if the patient is getting bifocals or progressives, since the near PD is typically 2-3 mm smaller than distance PD (eyes converge for near).
Fitting Heights for Multifocal Lenses
For bifocal, trifocal, or progressive addition lenses (PALs), the fitting point height (or seg height for bifocals) must be measured -- this is the vertical distance from the lower rim of the frame to the pupil center (or to where the progressive corridor begins). The frame must be properly fit to the patient's face before taking this measurement.
Frame Fitting Points
A properly fit frame should:
- Sit level on the face (both temples horizontal)
- Center the pupils horizontally in the lens opening
- Rest comfortably on the nose without sliding
- Not touch the eyelashes (minimum 10-12 mm clearance)
- Temple tips should rest comfortably behind the ear without pressure
- No pressure points on the nose, temples, or ears
Adjustments
Metal frames are adjusted with specialized tools (frame adjusting pliers, heat source for plastic frames). Common adjustments:
Pantoscopic Tilt
Pantoscopic tilt is the angle at which the frame tilts forward relative to the eye. The bottom of the lens tilts closer to the face. Normal pantoscopic tilt is 8-12 degrees. This angling positions the optical center correctly in front of the pupil for improved optical performance and is especially important for progressive lenses.
Face Form (Wrap) Angle
The degree to which the frame wraps around the face to follow its curvature. This affects the optical performance of the lens, particularly for high prescriptions or sports frames.
Vertex Distance
Vertex distance is the distance from the back of the lens to the front of the cornea (normally 12-14 mm). Significant changes in vertex distance affect the effective power of the lens, particularly for high prescriptions (above +/- 5.00 D). If a frame is fit with a different vertex distance than the exam was performed at, the prescription may need adjustment.
⚠️ Common Mistake: Measuring the fitting height or PD before the frame is properly adjusted on the patient's face leads to inaccurate optical centration. Always adjust the frame first, then take measurements.
Key Takeaways
- Frame size = eye size / bridge size; temple length measured separately
- PD is measured binocularly and monocularly; near PD is 2-3 mm less than distance PD
- Always fit the frame properly to the face before taking PD and fitting height measurements
- Pantoscopic tilt should be 8-12 degrees for optimal optical performance
- Vertex distance of 12-14 mm is standard; significant deviation affects effective prescription power
- Progressive lens fitting heights must be taken precisely while the patient wears the frame at its final fit