Eye health resource
What Is an Eye Refraction Test — and How Do We Refine Your Prescription?
A refraction determines the eyeglass prescription that gives you clear, comfortable vision. Learn how Park Slope Eye combines Topcon Chronos objective measurements with your own visual responses and the doctor's judgment.
Reviewed for accuracy by Justin Bazan, OD on .
“Which is clearer: 1 or 2?”
That familiar comparison is part of a refraction—the process an eye doctor uses to determine the lens prescription that gives you your clearest, most comfortable vision.
At Park Slope Eye, refraction is not just a machine printing a number and it is not just a long series of guesses. We use the Topcon Chronos Automated Binocular Refraction System to combine objective measurements with guided subjective comparisons, then the doctor reviews and refines the result before finalizing your prescription.
What Does a Refraction Measure?
A refraction measures refractive error: how the eye focuses light compared with where it needs to focus for clear vision.
Common refractive errors include:
- myopia (nearsightedness);
- hyperopia (farsightedness);
- astigmatism; and
- presbyopia, the age-related loss of near focusing ability.
The result helps your optometrist write an eyeglass prescription when correction is needed.
Why Park Slope Eye Uses the Topcon Chronos
Chronos combines several steps that traditionally require separate instruments. It can perform binocular autorefraction, keratometry measurements, visual-acuity testing, and subjective refraction in one system.
For the patient, the important benefit is not that a computer “replaces” the doctor. It is that we can gather precise objective information and your actual lens preferences in a consistent workflow, then use clinical judgment to confirm or modify the result.
Your final prescription is still a clinical decision. We consider the measurements, your responses, your current prescription, how you use your eyes, your visual comfort, and what makes sense for the way you actually live.
What Is Objective Refraction?
An objective refraction estimates your refractive error without requiring you to choose between lenses. Chronos can measure how light is focusing through each eye and provide an objective starting point.
It also measures corneal curvature through keratometry. Corneal shape is especially relevant to astigmatism and can provide useful context when we are evaluating the optical system of the eye.
Objective data are valuable because they give the doctor independent information before relying on your “1 or 2” responses.
What Is Subjective Refraction?
Subjective refraction is the part where your own visual experience matters. You compare lens choices and tell us which option looks clearer, more balanced, or more comfortable.
Chronos supports binocular subjective testing, so the refraction can include more than simply isolating one eye at a time and adding power until letters look smaller. The goal is to arrive at a prescription that works well for both eyes together.
Why Isn't the Strongest Lens Always the Best Prescription?
A refraction is not a contest to see how much minus or plus power we can add. A lens may make one line of letters look slightly sharper in the moment and still be unnecessarily strong, uncomfortable, or poorly balanced for everyday use.
The doctor therefore looks for the prescription that provides clear, comfortable, appropriate vision—not simply the maximum power you can tolerate during a short test.
Why Do My Answers Matter If the Machine Already Measured My Eyes?
Objective measurements are an excellent starting point, but vision is partly subjective. Two prescriptions that are numerically close can feel different, especially when astigmatism, binocular balance, adaptation, or a meaningful change from your old prescription is involved.
Your responses help us answer questions that an automated measurement cannot answer by itself: Which option actually looks better to you? Does one choice feel strained? Is the new prescription meaningfully better than what you already have? Are both eyes working comfortably together?
Can You Compare My Old Prescription With the New One?
Yes. One advantage of a digital refraction workflow is that we can consider your current correction alongside the new measurements rather than treating your old glasses as irrelevant.
Sometimes a prescription has changed enough that new glasses are likely to make a noticeable difference. Other times, a measured change is small and your current glasses may still be working well. A refraction should help answer that practical question—not simply produce new numbers.
What Do the Numbers on an Eyeglass Prescription Mean?
Prescription formats vary, but you may see several of these abbreviations:
- OD — right eye.
- OS — left eye.
- SPH — spherical lens power. Minus power is typically used for myopia and plus power for hyperopia.
- CYL — the amount of astigmatism correction.
- AXIS — the orientation of the astigmatism correction, written from 1 to 180 degrees.
- ADD — additional plus power used for near vision in multifocal prescriptions, commonly for presbyopia.
- PRISM — prism correction when prescribed for certain binocular-vision or alignment needs.
You may also hear about PD, or pupillary distance. PD is an eyewear measurement used to position lenses correctly in a frame. It is often measured as part of the eyeglass-ordering process and is not the same thing as the refractive prescription itself.
What Is Myopia?
Myopia, or nearsightedness, makes distant objects blurrier than near objects because light focuses in front of the retina instead of directly on it.
Myopia often begins during childhood and can progress while the eye is growing. Glasses and contact lenses correct the blur; children with progressive myopia may also be candidates for myopia-management treatment.
What Is Hyperopia?
Hyperopia, or farsightedness, occurs when the eye needs additional focusing effort because of its optical shape and power. Depending on age and degree, it can affect near vision, distance vision, or both.
What Is Astigmatism?
Astigmatism occurs when the eye's optical surfaces do not focus light equally in every direction. It commonly comes from the shape of the cornea and can also be influenced by the crystalline lens inside the eye.
CYL and AXIS describe the lens correction used to compensate for that unequal focusing power.
What Is Presbyopia?
Presbyopia is the normal age-related loss of near focusing ability. The crystalline lens inside the eye gradually becomes less flexible, making it harder to focus on close objects.
Reading glasses, bifocals, progressive lenses, or other multifocal options can provide the extra near power represented by the ADD portion of a prescription.
Does a Refraction Give Me a Contact Lens Prescription?
No. A refraction helps determine your spectacle prescription, but a contact lens prescription requires a separate contact lens evaluation and fitting.
Contact lenses sit directly on the eye, so the final prescription also depends on lens-specific information such as brand or material, base curve, diameter, design, and how the lens actually fits and performs on your eyes.
For a detailed comparison, see Eye Exam vs. Contact Lens Exam: What's the Difference?
Do I Have to Buy Glasses at Park Slope Eye to Get My Prescription?
No. When a refraction is performed, your eyeglass prescription is yours. There is no requirement to purchase eyewear from Park Slope Eye to receive it.
We do appreciate the opportunity to provide a quote if you need glasses. Our optical offers prescription eyewear in the office, and we often provide very competitive pricing, but you are free to use your prescription elsewhere.
For current self-pay exam and refraction pricing, see Insurance, Fees & Patient FAQs.
Refraction Is Only One Part of Eye Care
Clear vision does not necessarily mean the eyes are healthy. A comprehensive eye examination also evaluates eye health and can include testing or imaging based on your age, symptoms, risk factors, and clinical findings.
Learn more about comprehensive eye exams at Park Slope Eye or book an appointment online.
Originally published . Substantially updated .