Masters of Precision: Essential Measurements Every Optician Should Know

By Jon J. Trutt

Published: 3/31/2025

When it comes to making well-fitted eyeglasses, precision is everything. Whether it's aligning progressive lenses just right or ensuring a frame sits comfortably on a patient’s face, taking accurate measurements is a crucial skill for opticians. Below, we’ll break down six of the most important measurements—what they are, how to take them, and why they matter.

1. Segment Height (Seg Height)

Segment height is the vertical measurement from the bottom of the lens to the top of a bifocal or progressive addition lens segment. This ensures that the near-vision portion of the lens is positioned correctly for comfortable reading.

How to Measure:

• Have the patient wear the selected frame and adjust it for a natural fit.

• Ask the patient to look straight ahead in a relaxed posture.

• Using a PD ruler, measure from the lowest part of the lens inside the frame to the center of the patient’s lower eyelid (for bifocals) or pupil (for progressives).

Getting this measurement right is key—if the segment is too high, the wearer may have trouble with distance vision; too low, and they’ll have to tilt their head unnaturally to read.

2. Optical Center Height (OC Height)

The optical center of a lens is the point where light passes through without distortion. Properly aligning this point with the patient’s pupil is critical for clear, comfortable vision—especially for those with higher prescriptions, as even a small misalignment can cause unwanted prism effects.

How to Measure OC Height

• Fit the chosen frame on the patient, ensuring it sits naturally.

• Have the patient look straight ahead in a relaxed position.

• Mark the center of each pupil on the demo lenses while the patient is wearing the frame.

• Measure from the lowest point of the lens (inside the frame) up to this mark.

Using the Box Method for OC Height

The Box Method is a standardized way to measure lens dimensions, ensuring consistency in positioning. When measuring OC height using this method:

• Identify the B measurement, which is the total vertical height of the lens.

• Measure the distance from the bottom of the lens to the pupil center.

Adjusting for Larger Frames

Some frames are significantly larger, which means the bottom edge may not sit close to the eye—this can lead to an incorrect OC measurement if taken purely from the lowest lens point. To adjust for this:

• Consider how the frame sits on the patient’s face. If it rides lower due to its size, measuring directly from the bottom edge may result in an artificially high OC height.

• In such cases, measure the OC relative to the natural pupil position in the frame, not just the bottom lens edge, ensuring the optical center aligns with the actual wearing position.

By taking these factors into account, opticians can ensure accurate OC placement, leading to sharper vision and a more comfortable experience for the wearer.

3. Pupillary Distance (PD)

Pupillary distance is the horizontal measurement between the centers of the pupils. This measurement ensures that the optical centers of the lenses align properly with the eyes, reducing strain and distortion.

How to Measure:

• Use a PD ruler or a digital measuring device.

• For distance PD, have the patient focus on a distant object and measure from the center of one pupil to the center of the other.

• For near PD, repeat the measurement while the patient looks at a near object, as their pupils will naturally converge.

Most prescriptions are based on distance PD, but for reading glasses, near PD is used to position the lenses for optimal close-up vision.

4. Lens Height (B Measurement)

Lens height (or the B measurement) refers to the vertical dimension of the lens, from the highest to the lowest point. This is particularly important when selecting frames for progressive lenses, as they require enough vertical space to accommodate all prescription zones.

How to Measure:

• Measure from the top edge of the lens to the bottom edge, ensuring the frame provides enough height for the desired lens design.

A frame that’s too short may not allow for a proper progressive corridor, leading to an uncomfortable wearing experience.

5. Face Form Angle

The face form angle, or frame wrap, describes how much the front of the frame curves around the face. A slight curve improves peripheral vision and enhances the overall fit, especially for high-prescription lenses or wraparound sports eyewear.

How to Measure:

• Observe the angle at which the frame sits on the face.

• Ideally, the frame should have a mild wrap, aligning with the natural curve of the wearer’s face.

If a frame is too flat, the patient may experience unwanted peripheral distortion. If it wraps too much, the lenses may need additional prism adjustments to compensate.

6. Frame Width

Frame width is the total horizontal measurement of the frame from end piece to end piece. Getting this right ensures a proper fit—too wide, and the glasses will slide down; too narrow, and they’ll pinch the temples.

How to Measure:

• Measure the total width of the frame, including the end pieces.

• Compare this to the patient’s face width to ensure a balanced fit.

Proper frame width ensures both comfort and a polished, well-proportioned look.

Taking precise measurements isn’t just about numbers—it’s about ensuring every patient gets a pair of glasses that not only helps them see clearly but also fits comfortably and looks great. Mastering these six key measurements will set any optician up for success in delivering high-quality, custom-tailored eyewear.

If you ever have any questions about measurements, please contact AccuLab at 800-688-3904 or info@acculab.net.