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Practice questions and plain-English guides for the ABO, NCLE, and COA exams.

Original questions written for opticians and ophthalmic assistants, with full explanations, optics calculators, and a state licensing guide. Free, no account.

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Independent: not affiliated with ABO-NCLE or IJCAHPO. Last reviewed Oct 8, 2026.

Sample question · Ophthalmic Optics · Prentice’s Rule · Beginner

A patient wears a +3.00 D spherical lens. Their pupil sits 4 mm away from the lens optical center. How much prism is induced at the pupil?

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Answer: B, 1.20Δ

Prentice’s Rule: prism induced by decentration A +3.00 D plus lens with its optical center on the horizontal axis. The pupil looks through a point 4 millimeters (0.4 centimeters) above the optical center, so the ray is bent toward the optical center. Prism equals 0.4 cm times 3.00 diopters, which is 1.20 prism diopters, base down in this example. optical center pupil c = 4 mm base down 1.20Δ

Δ = c × F = 0.4 cm × 3.00 D = 1.20Δ

Shown with the pupil above the optical center of a +3.00 D lens.

Prentice's Rule: prism (Δ) = decentration in centimeters × lens power in diopters. Convert 4 mm to 0.4 cm, then 0.4 × 3.00 = 1.20Δ. The classic mistake is leaving decentration in millimeters (4 × 3 = 12), which overstates the prism tenfold. Base direction depends on lens sign: a plus lens behaves like two prisms base-to-base, so the base points toward the optical center.

Read the Prentice’s Rule guide

The three exams

Formats from ABO-NCLE and IJCAHPO. Each guide links to the official source.

Exam Who it’s for Format Content areas Study
ABO Basic NOCE Opticians who dispense eyeglasses 125 q · 2 h 6 domains: optics, products, dispensing, standards Guide Practice FAQ
NCLE Basic CLRE Opticians and technicians who fit contact lenses 125 q · 2 h 8 domains: prefitting, fitting, dispensing, follow-up Guide Practice FAQ
COA® IJCAHPO Ophthalmic assistants working with an ophthalmologist 200 q · 180 min 22 content areas, history-taking to tonometry Guide Practice FAQ

Study topics

Guides with worked examples, each paired with practice questions.

See the optics

Two concepts every candidate meets, as diagrams you can operate.

How a lens bends light

Light passing through a lens Plus (convex) lens: parallel rays bend toward the axis and converge at the focal point F behind the lens. F F
Plus (convex) lens: parallel rays bend toward the axis and converge at the focal point F behind the lens.

The duochrome (red-green) test

Duochrome test chart Equal on both sides: the red and green images straddle the retina evenly. This is the balanced endpoint.
Equal on both sides: the red and green images straddle the retina evenly. This is the balanced endpoint.

Calculators

Does your state license opticians?

The ABO-NCLE licensing-board directory lists 21 states plus Puerto Rico that license opticians. Search our state guide for the board name, the official link, and how each entry was sourced.