What is the A dimension of an eyeglass frame?

The A dimension (also called eye size) is the horizontal width of the lens opening at its widest point, measured in millimeters. It is part of the boxing system of frame measurement. Combined with the bridge size (DBL), it gives the frame PD (A + DBL), which is used to calculate the decentration needed for lens centering.

What is the effective diameter (ED) of a frame?

The effective diameter is the longest straight line from the geometric center of the lens opening to the farthest edge point, doubled. It represents the maximum dimension a lens must span to fill the frame opening. ED is the critical measurement for calculating minimum blank size. For irregular or elongated frame shapes, the ED can be significantly larger than the A dimension.

How do you calculate frame PD?

Frame PD equals the A dimension (eye size) plus the DBL (bridge size). For example, a frame with a 54mm eye size and 17mm bridge has a Frame PD of 71mm. This value is compared to the patient's PD to determine the decentration required for each lens: Decentration per eye = (Frame PD - Patient PD) / 2.

What is the minimum B dimension for progressive lenses?

Most progressive lens designs require a minimum B dimension (vertical depth) of 28 to 30 mm, though this varies by lens design. Short-corridor progressives are available for frames as small as 25-26 mm. Always check the specific progressive design's fitting requirements before selecting a frame, as insufficient vertical depth means the patient will not have access to the full range of the progressive corridor.

Frame Gauge: Boxing System Measurements for Spectacle Frames

What Is a Frame Gauge?

A frame gauge is a measuring tool used to determine the precise dimensions of an eyeglass frame opening. These measurements, expressed in the boxing system, are essential for ordering the correct lens blanks, calculating decentration, and ensuring proper lens fit.

The Boxing System

The boxing system is the standard method for measuring spectacle frame dimensions. It works by drawing an imaginary rectangle (a "box") around the lens opening at the tightest fit. All measurements reference this box.

A Dimension (Eye Size)

The A dimension is the horizontal width of the box, which equals the widest horizontal measurement of the lens opening. This is commonly called the "eye size." Typical values range from 44 mm to 62 mm.

B Dimension

The B dimension is the vertical height of the box, which equals the tallest vertical measurement of the lens opening. This measurement is important for progressive lens fitting, as the B dimension must be deep enough to accommodate the progressive corridor. Minimum B dimension for progressives is typically 28-30 mm, depending on the design.

DBL (Distance Between Lenses)

DBL is the shortest distance between the two lens openings, measured at the narrowest point of the bridge. This is commonly referred to as the bridge size. Typical values range from 14 mm to 24 mm.

Effective Diameter (ED)

The effective diameter is the longest straight line that can be drawn from the geometric center of the lens opening to the farthest edge, doubled to give a full diameter. This is the most critical measurement for calculating minimum blank size.

Measurement	Definition	Typical Range	Primary Use
A dimension	Horizontal width of lens opening	44-62 mm	Frame sizing, PD calculation
B dimension	Vertical height of lens opening	28-52 mm	Progressive suitability
DBL	Shortest distance between lens openings	14-24 mm	Bridge fitting
ED	Longest dimension from geometric center x 2	50-72 mm	Minimum blank size

The frame PD is calculated as A + DBL. This value compared to the patient's PD determines the required decentration. Frame PD = Eye Size + Bridge Size. Example: A frame with a 52mm eye size and 18mm bridge has a Frame PD of 70mm.

Using the Frame Gauge

The frame gauge is a specialized ruler or caliper designed to fit into the lens opening of a frame. To measure:

A dimension: Measure the widest horizontal point of the lens opening
B dimension: Measure the tallest vertical point of the lens opening
DBL: Measure the shortest distance between the inner edges of the two lens openings
ED: Find the geometric center of the opening, then measure to the farthest edge point and double it

Some frame gauges have built-in features for measuring temple length and other dimensions. Digital frame gauges provide measurements electronically.

Most modern frames have their measurements printed on the temple: eye size □ bridge - temple length (e.g., 52 □ 18 - 140). Use the frame gauge to verify these printed measurements, as manufacturing variations can cause slight differences from the marked values, especially in hand-made acetate frames.

Geometric Center

The geometric center (GC) of the lens opening is the center point of the bounding box (A/2 horizontally, B/2 vertically). The geometric center is NOT the same as the optical center of the lens. The optical center is determined by the patient's PD and seg height, while the geometric center is a fixed property of the frame.

The distance between the geometric center and the optical center is the decentration.

Datum System vs. Boxing System

While the boxing system (described above) is the standard used in the United States, an older system called the datum system exists. The datum system uses the horizontal midline of the lens opening as a reference. The boxing system's A measurement is always larger than the datum system's measurement for non-rectangular frame shapes. The ABO exam uses the boxing system.

Confusing the A dimension with the ED. The A dimension is the horizontal width of the lens opening. The ED is the longest dimension from center to edge, doubled. For round or square frames, the ED may equal the A dimension. For frames with an elongated or irregular shape, the ED may be significantly larger than the A dimension because the farthest point from center is not on the horizontal axis.

Clinical Relevance

Frame gauge measurements are the starting point for all lens ordering calculations. Incorrect measurements lead to wrong blank sizes, improper decentration, and lenses that do not fit the frame. Accurate frame measurement is the foundation of competent dispensing.

Key Takeaways

The boxing system uses A dimension (eye size), B dimension (vertical depth), DBL (bridge), and ED (effective diameter)
Frame PD = A + DBL, used to calculate decentration
ED is the critical measurement for minimum blank size calculation
The B dimension determines suitability for progressive lenses (minimum 28-30 mm)
The geometric center of the frame is the center of the bounding box, not the optical center
Verify printed frame measurements with a gauge, as manufacturing variations occur