Pantoscopic Tilt, Wrap (Face Form), and Vertex: How Frame Fit Changes Lens Performance

In Brooks and Borish’s terms, pantoscopic tilt is the lower rims tipping in toward the cheeks, face form is the frame front bending around the face, and vertex distance is the lens-to-cornea gap. Brooks and Borish note that tilt changes where the optical center belongs, and a vertex change alters the effective power of stronger lenses.

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Terms

These definitions follow Brooks and Borish:

• Pantoscopic tilt: the lower rims tip toward the cheeks.
• Face form: the frame front wraps around the face; also called wrap.
• Vertex distance: from the back surface of the lens to the front of the cornea.

Why tilt affects OC placement

Brooks and Borish describe a tilted lens as working best when its optical axis runs through the eye’s center of rotation, which means placing the optical center below the pupil. Their rule of thumb is 0.5 mm lower for each degree of tilt, or 1 mm per 2 degrees.

• 8° tilt: 8 × 0.5 = 4 mm below the pupil center.
• 12° tilt: 12 × 0.5 = 6 mm below.

Vertex

A lens’s effect depends on where it sits. Move a strong lens closer to or farther from the eye and its effective power changes, so the power must be recalculated. Example: −8.00 D at 12 mm, moved to the eye: −8.00 ÷ (1 + 0.096) = −7.30, or −7.25 D to the nearest quarter. Use the vertex distance calculator and the vertex distance guide.

Adjusting order

Brooks and Borish advise fitting and adjusting the frame to how it will be worn first, then taking heights and other measurements. A frame that later moves on the face moves every measurement with it.

Practice set

Original problems.

1. The frame has 10° of pantoscopic tilt. How far below the pupil center should the OC go?

   Show answer

   Using the Brooks and Borish rule: 10 × 0.5 = 5 mm.

2. 6° of tilt. OC placement?

   Show answer

   Using the Brooks and Borish rule: 6 × 0.5 = 3 mm below the pupil center.

3. Which measurement is the gap between lens and cornea?

   Show answer

   Vertex distance.

4. +10.00 D glasses at 12 mm. Power at the cornea, to the nearest 0.25 D?

   Show answer

   10.00 ÷ (1 − 0.12) = 11.36 → +11.25 D.

5. You measure heights, then bend the frame to add tilt. What now?

   Show answer

   Measure again. Brooks and Borish advise adjusting first, then measuring.

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Standard text used for this page: Brooks CW, Borish IM. System for Ophthalmic Dispensing, 3rd ed. Butterworth-Heinemann, ch. 4 and 5 (tilt, face form, vertex, optical center height rule, adjust before measuring) and ch. 14 (effective power). ABDO’s 2024 measurement review and the Iowa optics review are linked under sources.

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