6.3 Frame Selection for Prescriptions

6.3 Frame Selection for Prescriptions

The Optics of Frame Selection

While patients often select frames based on fashion, the optician's primary responsibility is to ensure that the chosen frame is optically compatible with the patient's prescription. A poor frame choice can result in thick, heavy lenses, poor visual performance, and intolerance to the eyewear. The key to successful optical dispensing is understanding how lens shape, frame size, and eye positioning affect the final thickness, weight, and optical clarity of the lenses.

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Decentration and Its Optical Consequences

Decentration is the displacement of the optical center of a lens from the geometric center of the frame's eyewire. Ideally, the optical center of the lens must line up directly with the patient's pupil to prevent unwanted prismatic effects.

To calculate the total decentration of a pair of glasses, we must first determine the Frame PD (also called the Frame Center Distance, FCD):

Frame PD = A + DBL

where A is the horizontal lens width and DBL is the distance between lenses (bridge width).

The total decentration is the difference between the Frame PD and the patient's Pupillary Distance (PD):

Total Decentration = Frame PD - Patient PD

To find the decentration required for each individual lens:

Decentration per Eye = (Frame PD - Patient PD) / 2

Why Minimizing Decentration Matters

• Lens Blank Size: The lab must select a raw lens blank large enough to cover the frame shape after decentering the optical center. Larger decentration requires a larger lens blank.
• Lens Thickness: For minus lenses, larger decentration leads to thicker temporal edges. For plus lenses, larger decentration leads to a thicker nasal center.
• Weight: Excess thickness directly translates to excess weight, causing the glasses to slide down the nose and cause discomfort.
• Aberrations: Looking through the peripheral portions of a highly decentered lens increases chromatic aberration and distortion.
• Rule of Thumb: The ideal frame has a Frame PD that is equal to or very close to the patient's PD, resulting in zero or minimal decentration.

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High Minus Prescriptions

Minus lenses are thin in the center and thickest at the edges. The power of a minus lens increases toward the periphery, meaning edge thickness increases exponentially with the size of the lens.

Selection Strategies for High Minus (e.g., above -4.00 D)

1. Choose Small Frame Sizes (A Measurement): Reducing the horizontal lens width from 55mm to 48mm can reduce edge thickness by up to 30%.
2. Opt for Round or Oval Shapes: Round shapes have a smaller Effective Diameter (ED) compared to rectangular or aviator shapes. Square or rectangular frames have sharp corners that extend far from the center, which catches the thickest, heaviest part of the minus lens.
3. Select Thick Plastic (Zyl) Frames: Plastic eyewires have a thicker profile than thin metal wires, which helps hide the visible edge of the lens.
4. Avoid Rimless and Semi-Rimless Styles: The exposed, thick edges of a high-minus lens in a semi-rimless or rimless frame look aesthetically unappealing and are prone to chipping.

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High Plus Prescriptions

Plus lenses are thickest in the center and thin at the edges.

Selection Strategies for High Plus (e.g., above +4.00 D)

1. Choose Small, Round Frames: Plus lenses are surfaced from a blank where the center thickness is determined by the minimum edge thickness at the widest point. A small, round frame allows the lab to use a very small, thin lens blank, minimizing both center thickness and weight.
2. Minimize Decentration: High decentration in a plus lens results in an excessively thick center, creating a heavy lens that causes a "magnified eye" (bug-eye) effect.
3. Maintain a Short Vertex Distance: Fitting the frame close to the eye decreases the magnification effect and increases the patient's usable field of view.

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Frame Selection for Progressive Lenses

Progressive Addition Lenses (PALs) provide a seamless transition from distance vision (top of the lens) to near vision (bottom of the lens) through an intermediate corridor.

Corridor Depth and the B Measurement

The vertical height of the frame eyewire is known as the B measurement.

• Traditional PALs: Require a minimum B measurement of 28mm to 30mm (or a fitting height of at least 18-20mm from the pupil to the bottom rim) to ensure the entire reading zone is not cut off during the lens edging process.
• Short-Corridor PALs: Designed for smaller frames, these lenses compress the intermediate zone, allowing fitting heights as low as 14mm to 16mm (with a B measurement of 22-24mm). However, the trade-off is a narrower intermediate corridor and a faster transition, which some patients find difficult to adapt to.
• Shape Considerations: Avoid aviator styles or frames with drastically sloped bottom rims. The progressive corridor runs downward and slightly nasal. A sloped bottom-nasal corner will cut off the reading zone.

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Prescription-Based Frame Selection Guidelines