Refraction — the process of determining the optical correction needed to bring the retinal image into sharp focus — is central to ophthalmology practice. While the physician or optometrist performs the subjective refraction, the ophthalmic assistant plays a critical role: operating the autorefractor, preparing the phoropter, understanding the prescription components, and assisting with the overall workflow. The COA exam tests your knowledge of refractive concepts extensively, as refractometry is one of the highest-weighted sub-topics within the Assessments domain.
This guide covers the refractive errors you need to understand (myopia, hyperopia, astigmatism, presbyopia), how to read and interpret optical prescriptions, the role of the autorefractor in clinical practice, and the distinction between manifest and cycloplegic refraction — all key knowledge areas for the COA exam.
The Four Major Refractive Errors
Myopia (Nearsightedness)
Light focuses in front of the retina. The eye is too long (axial myopia) or has too much refractive power (refractive myopia).
- Blurry distance vision; clear near vision
- Corrected with minus (concave) lenses
- Negative sphere in prescription (e.g., -3.00)
- Prevalence increasing worldwide
Hyperopia (Farsightedness)
Light focuses behind the retina. The eye is too short or has insufficient refractive power.
- Blurry vision at all distances (when severe)
- Young patients compensate with accommodation
- Corrected with plus (convex) lenses
- Positive sphere in prescription (e.g., +2.50)
Astigmatism
The cornea or lens has different curvatures in different meridians, like a football rather than a basketball.
- Blur at all distances; distortion and ghosting
- Requires cylindrical correction
- Three components: Sph + Cyl x Axis
- Can coexist with myopia or hyperopia
Presbyopia
Age-related loss of accommodative amplitude. The crystalline lens stiffens and can no longer change shape to focus at near.
- Near vision deteriorates, typically after age 40
- Corrected with reading glasses, bifocals, or multifocals
- Add power appears as "+Add" in prescription
- Both eyes affected equally (bilateral)
Reading an Ophthalmic Prescription
A standard spectacle prescription has a defined format. Understanding each component is essential for ophthalmic assistants who transcribe, verify, and transmit prescriptions daily. The COA exam frequently tests the ability to interpret prescription values and relate them to the patient's refractive condition.
| Component | Abbreviation | Units | What It Means |
|---|---|---|---|
| Sphere | Sph | Diopters (D) | Spherical power: negative = myopia, positive = hyperopia |
| Cylinder | Cyl | Diopters (D) | Cylindrical power to correct astigmatism (can be + or -) |
| Axis | x or Ax | Degrees (1-180) | Orientation of cylinder correction meridian |
| Add power | +Add | Diopters (D) | Additional plus power for near vision (presbyopia correction) |
| Prism | Prism / Base | Prism diopters (Δ) | Prismatic correction for eye alignment (strabismus/binocular vision) |
| Pupillary distance | PD | Millimeters (mm) | Distance between pupils; needed to center lenses correctly in frame |
Example Prescription Interpretation
OS: -2.50 -0.75 x 010 Add: +2.00
- OD: Right eye has 3.25D of myopia plus 1.75D of astigmatism with the cylinder at 165 degrees
- OS: Left eye has 2.50D of myopia plus 0.75D of astigmatism with the cylinder at 10 degrees
- +2.00 Add: Same reading addition for both eyes — this patient has presbyopia and likely wears bifocals or progressive lenses
- Overall: This is a myopic patient with bilateral astigmatism and presbyopia — common in adults over 45
The Autorefractor: Operation and Limitations
The autorefractor (also called an automated refractor or ARK in some practices) provides an objective, instrument-based estimate of refractive error before the physician performs the subjective refraction. It works by projecting an infrared beam into the eye and analyzing the wavefront of the reflected light to calculate sphere, cylinder, and axis. Modern autorefractors complete a measurement in less than a second and typically average three or more measurements for each eye automatically.
Patient positioning
Seat the patient at the autorefractor with their chin in the chin rest and forehead against the headband. Align the instrument to the patient's eye using the joystick or touch controls. The patient's eye should be centered in the instrument's viewfinder and the pupil clearly visible.
Fixation and fogging
Instruct the patient to look at the fixation target inside the instrument — usually a house, balloon, or balloon on a string at the end of a lane. Modern autorefractors use "fogging" (defocusing the target slightly) to relax accommodation and reduce instrument myopia — the tendency for accommodation to stimulate myopic readings in the instrument-near environment.
Obtain and print measurements
Take at least three measurements per eye (most autorefractors do this automatically). Print the readout and attach it to the patient's chart or enter the values into the EHR. Present the printout to the examining physician or optometrist before the refraction. The autorefractor measurement is a starting point, not a final prescription.
Limitations of Autorefractors
Autorefractors have important limitations that the COA exam may test: (1) Instrument myopia — the near fixation distance can stimulate accommodation, causing artificially myopic readings. (2) Corneal irregularities (keratoconus, post-LASIK) reduce accuracy because the instrument assumes a spherocylindrical cornea. (3) Dense cataracts, corneal opacities, and very small pupils reduce the quality and reliability of measurements. (4) Children with high accommodation may require cycloplegic drops before autorefraction to get a reliable result.
Manifest vs. Cycloplegic Refraction
Manifest Refraction
Performed without cycloplegic drops. The patient's accommodation remains active. This is the standard refraction for most adult patients and produces the prescription for daily wear correction.
Used for routine refraction in adults
No drops needed; patient can drive afterward
Reflects prescription used in daily activities
May miss latent hyperopia in young patients
Cycloplegic Refraction
Uses cycloplegic drops (cyclopentolate 1%, atropine 1%) to paralyze the ciliary muscle and fully relax accommodation. Required when accommodation interferes with accurate measurement.
Essential in children (strong accommodation)
Reveals full (manifest + latent) hyperopia
Used pre-operatively and in convergent strabismus
Blurred near vision for hours after; cannot drive
Practice COA Refraction Questions
Opterio covers refraction, prescription interpretation, and all COA assessment skills with adaptive practice and AI-powered explanations for every answer.
Spherical Equivalent: When and Why
Spherical equivalent (SE) collapses a spherocylindrical prescription into a single sphere power. The formula is SE = Sph + (Cyl/2). This calculation is frequently tested on the COA exam.
SE Calculation Examples
-3.00 -1.50 x 090
SE = -3.00 + (-1.50/2) = -3.00 + (-0.75) = -3.75 SE
+1.50 -0.50 x 180
SE = +1.50 + (-0.50/2) = +1.50 + (-0.25) = +1.25 SE
-2.00 +1.00 x 045
SE = -2.00 + (+1.00/2) = -2.00 + (+0.50) = -1.50 SE