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Study guides

Plain-English explainers with worked examples. Each guide links to a calculator or practice set so you can check your understanding.

ABOOptics math

Prentice's Rule

Calculate induced prism from lens power and decentration, find the base direction, and work out vertical imbalance.

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ABOCOAPrescriptions

Lens transposition

Convert prescriptions between plus- and minus-cylinder form, read a power cross, and find the spherical equivalent.

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NCLECOAInstruments

Keratometry basics

What K readings measure, how to convert millimeters to diopters, and how to describe corneal astigmatism.

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COAStudy plan

COA study plan

Plan COA prep around IJCAHPO’s 22 weighted content areas, with an example 8-week schedule and pacing math.

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See the optics

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.

More guides on the way

Next up: vertex distance, PD and segment height, lens materials and coatings, multifocal basics, contact lens terminology, and infection control. Have a request? Let us know.

Independent study resource

Not affiliated with or endorsed by ABO-NCLE, IJCAHPO, or any state board. Not medical advice. Practice questions are original and don't guarantee a passing score. Verify current requirements with the official source. Read the full disclaimer.