Frame Selection and Spectacle Fitting: Boxing System, PD, Fitting Height, Tilt and Wrap
Last updated: November 2026
RPC Rounds for Optometrists · B.Optom study guide
A good prescription can still fail in a badly chosen or badly fitted frame. Frame choice affects lens thickness, weight, field of view and where the optical centres sit. Learn the boxing measurements, how to match the frame to the prescription and face, and the standard adjustments.
The boxing system (frame measurements)
| Measurement | Meaning |
|---|---|
| Eye size (A) | Horizontal width of the lens box (for example 52 in "52□18 140") |
| B measurement | Vertical depth of the lens box |
| Bridge (DBL) | Distance between lenses (for example 18 mm) |
| Temple length | Overall length of the side (for example 140 mm) |
| Frame PD (geometric centre distance) | A + DBL: the distance between the box centres |
| Effective diameter (ED) | Twice the longest radius from the box centre: decides the uncut lens blank size |
Decentration per lens = (frame PD − patient PD) ÷ 2. A large decentration needs a bigger lens blank and thickens the edge of minus lenses or the centre of plus lenses.
Choosing a frame for the prescription
| Prescription | Frame choice |
|---|---|
| High minus | Small, round or oval eye size; frame PD close to the patient's PD; thick rims or acetate hide edge thickness; avoid rimless |
| High plus | Small, round frame to keep centre thickness low; aspheric lens |
| Progressive | Adequate depth (enough height below the pupil for the corridor and near zone); adjustable pads helpful |
| High astigmatism | Avoid very wide frames; keep vertex short and stable |
| Children | Polycarbonate or Trivex lenses, flexible frames, correct bridge fit (children have low, flat bridges), cable or sports temples for toddlers |
| Rimless | Polycarbonate or Trivex; moderate powers only |
Frame materials
- Metal: monel, stainless steel, titanium (light, strong, hypoallergenic), memory metal (flexible); adjustable pads.
- Plastic: cellulose acetate (most common, adjustable with heat), nylon and TR-90 (flexible, light), carbon fibre.
- Nickel allergy is common: suggest titanium or plastic.
Measurements to take
- Distance PD (binocular or monocular from the centre of the bridge) with a PD rule or pupillometer; near PD is about 3–4 mm less.
- Fitting heights (for bifocals, progressives, high-power aspherics) with the frame adjusted on the face.
- Vertex distance for powers over ±4.00 D; pantoscopic tilt (about 8–12°) and face form (wrap).
Fitting and adjusting the frame
- Frame front level and centred; lenses sit with tilt of about 8–12°; lashes do not touch the lenses.
- Bridge or pads distribute weight evenly; no pressure marks on the nose.
- Temples run straight back without pressing the head; bend down behind the ear at the right point, touching the skull behind the ear evenly.
- Check the frame is the same distance from both eyes (vertex) and that the optical centres match the pupils.
| Problem | Adjustment |
|---|---|
| Frame slides down | Tighten temple bends or adjust pads; check bridge size |
| One lens higher than the other | Bend one temple down at the hinge or adjust pad arms (ears are rarely level) |
| Lashes touch the lenses | Increase vertex distance (pads out) or reduce tilt |
| Pressure behind ears | Move the bend further back or loosen the curl |
Practical pearls
- Fit the frame on the face before measuring heights: measurements in an unadjusted frame are wrong once you adjust it.
- For high minus prescriptions, choose a frame whose geometric centre distance is close to the patient's PD to avoid thick temporal edges.
- Children grow out of frames quickly; choose a frame that fits now, not one to grow into, or the optical centres will be wrong.
Where students slip
- Using a large fashion frame for a −8.00 D prescription.
- Measuring progressive heights with the patient looking down at you.
- Forgetting near PD for single-vision reading glasses.
Test yourself: flip cards
Quick quiz
For daily counter use, see the lens types comparison and order checklist and the sunglasses, polarised and photochromic lens guide.
Related: Spectacle Dispensing Reference: PD, Fitting Height, Tolerances, Spectacle Lens Designs: Aspheric, Bifocals and Progressive Lenses (Fitting and Troubleshooting), Spectacle Lens Materials and Coatings: CR-39, Polycarbonate, Trivex, High Index, Abbe Value and Blue-Cut, Your Child's First Glasses: Choosing Frames and Helping Them Adjust, Spectacle Non-Tolerance: A Step-by-Step Troubleshooting Guide for Optometrists. Back to the optometry curriculum.
Key sources
- Brooks CW, Borish IM. System for Ophthalmic Dispensing. 3rd ed. Butterworth-Heinemann; 2007
- Jalie M. Ophthalmic Lenses and Dispensing. 3rd ed. Butterworth-Heinemann; 2008
- ISO 8624: Ophthalmic optics, spectacle frames, measuring system and terminology (boxing system)
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