ABO Exam Glossary

How to use this glossary

This glossary is written for ABO-style questions, not for textbook completeness. Each entry focuses on the meaning and usage that you’re most likely to see on the exam.

Many of these terms also appear throughout the Questions and Calculators pages, so you’ll see them used in context as you practice.

• Scan alphabetically for a term that appears in a question.
• Use the definition to anchor what the question is really asking.
• Then apply the related formula or concept on your own.

Key terms (A–M)

A size

The horizontal (eye) size of the lens opening in the frame, measured at its widest point. Used in frame PD and MBS calculations.

Add power

The extra plus power added to the distance prescription to create the near portion of a multifocal lens. Labeled “ADD” in the Rx.

Base curve

The primary curve on the front surface of the lens, chosen to balance optical performance and cosmetics. On the exam, base curve questions often ask whether a finished job matches the specified base curve within a tolerance.

DBL (Distance Between Lenses)

The distance between the inner edges of the lens openings (“bridge size”), measured at the closest point. Used with A size to calculate the frame PD.

ED (Effective Diameter)

The longest radius from the geometric center of the lens to the farthest edge, doubled. Used in minimum blank size (MBS) calculations to ensure full cutout.

Frame PD

The frame’s pupillary distance, calculated as A size + DBL. Compared with the patient’s distance PD to find decentration.

Gerstman’s three-quarter rule

A rule of thumb for estimating near inset: inset per eye (mm) ≈ 0.75 × dioptric demand (D). Helps convert working distance into a practical near PD.

MBS (Minimum Blank Size)

The smallest lens blank that can be edged to fit the frame while keeping all necessary optics inside the lens. A common approximation is: MBS ≈ ED + 2 × decentration per eye + safety factor.

Monocular PD

The distance from the center of the bridge to the center of one pupil. Two monocular PDs (right and left) can be added to get the binocular PD.

Key terms (N–Z)

Near PD

The pupillary distance used for near tasks. Typically smaller than distance PD because the eyes converge at near. Can be estimated with Gerstman’s three-quarter rule.

Prentice’s Rule

The formula for calculating induced prism: Δ = c × F, where Δ is prism diopters, c is decentration in centimeters, and F is lens power in diopters.

Prism diopter (Δ)

A unit describing how much a prism deviates light. One prism diopter displaces an image 1 cm at a distance of 1 meter from the prism.

Segment height

The vertical distance from a reference point on the frame (often the deepest point or a specified line) to the top of the multifocal segment. Used when ordering bifocals and progressives.

Safety factor

Extra millimeters added to the minimum blank size calculation to account for edging, alignment, and minor measurement error—often around 2 mm in ABO-style examples.