A complete refraction follows a predictable sequence: start with an objective measurement, refine it with the patient’s input, adjust for astigmatism, then balance both eyes together. The whole process typically takes 5 to 15 minutes once you’re comfortable with each step. Here’s the full sequence from start to finish.
Step 1: Get a Starting Point
Every refraction begins with an objective measurement, meaning one that doesn’t depend on the patient’s responses. The two most common methods are autorefraction (a machine-generated reading) and retinoscopy (a handheld technique). Many clinicians use both, but you need at least one to give you a reliable starting prescription to refine.
In retinoscopy, you shine a streak of light into the patient’s eye and watch how the reflected light (the “reflex”) moves inside the pupil. The direction of that movement tells you what kind of refractive error you’re dealing with:
- “With” motion: The reflex moves in the same direction as your streak. This indicates hyperopia, emmetropia, or low myopia.
- “Against” motion: The reflex moves opposite to your streak. This indicates myopia greater than the inverse of your working distance.
- “Neutral” motion: The reflex fills the pupil with no clear directional movement. This is your endpoint.
You add lenses until you reach neutrality, then subtract your working distance in diopters. If you’re sitting 67 cm from the patient, your working distance is about 1.50 diopters, so you subtract 1.50 from whatever lens power achieved neutrality. That gives you the patient’s approximate prescription for that eye. Repeat for the other eye.
Step 2: Fog the Eye to Relax Focus
Before refining, you need to prevent the patient’s focusing muscles from tightening up and skewing results. This is called “fogging.” You add extra plus power (typically +0.75 diopters) over the objective starting point, which intentionally blurs the patient’s vision by about one to two lines on the eye chart. The blur relaxes the eye’s automatic focusing reflex, giving you a more accurate baseline to work from.
If the patient can still read nearly as well as before you added the fog, they haven’t relaxed enough. Add more plus until they’re genuinely blurred.
Step 3: Find the Best Starting Sphere
This step is often called “maximum plus to maximum visual acuity,” or MPMVA. With one eye occluded, slowly reduce the plus power (or add minus) in 0.25 diopter steps. Ask the patient to read the chart after each change. Expect roughly one line of improvement for every 0.25 diopters you add.
Stop as soon as the patient reaches their best acuity, typically 20/20 or 20/15. The key principle: for a nearsighted patient, prescribe the weakest minus lens that gives best vision. For a farsighted patient, prescribe the strongest plus lens that gives best vision. This prevents over-correction, which can cause eyestrain and headaches.
Step 4: Refine the Cylinder Axis
Now you address astigmatism. Astigmatism is always corrected before finalizing the sphere, because changing the cylinder affects the overall clarity of the image. You’ll use a Jackson Cross Cylinder (JCC), a lens that flips between two orientations.
To refine the axis, align the JCC handle with the axis of the correcting cylinder already in place. Flip the JCC and ask the patient: “Which is clearer, option one or option two?” Rotate the correcting cylinder’s axis toward the red dot on whichever side the patient prefers. Flip again and repeat. You’re done when the patient says both options look the same, or when the preference reverses direction. At that point, you’ve bracketed the correct axis.
Step 5: Refine the Cylinder Power
Next, keep the JCC in front of the eye but change its alignment. Now place the JCC so its power axis runs parallel to the correcting cylinder’s axis. Flip between the two positions and ask the same “one or two” question.
- If the patient prefers the side with the red dot over the correcting lens, increase the cylinder by 0.25 diopters.
- If they prefer the side with the black (or white) dot, decrease the cylinder by 0.25 diopters.
- If both look the same, the cylinder power is correct and you can stop.
Each time you change the cylinder power by 0.25 diopters, compensate the sphere by half that amount in the opposite direction. So if you add -0.25 cylinder, add +0.125 sphere (round to the nearest 0.25). This keeps the overall focus point stable while you isolate the astigmatism correction.
Step 6: Re-Check the Sphere
After adjusting the cylinder, go back and fine-tune the sphere one more time. Add plus in 0.25 diopter steps until the patient just begins to notice blur, then back off to the last clear setting. This confirms you haven’t drifted into over-minus territory during the cylinder refinement. Remove the JCC from in front of the eye and ask the patient to read the smallest line they can.
Step 7: Repeat for the Other Eye
Occlude the first eye and run through steps 2 through 6 on the second eye. The process is identical.
Step 8: Binocular Balancing
With both eyes now individually refined, you need to make sure the two eyes are balanced so neither one is working harder than the other. Start by adding +0.75 diopters to both eyes, blurring vision to around 20/30 or 20/40. The mild blur is important because the balancing technique doesn’t work well at peak acuity.
The most common method uses prism dissociation. Set 3 prism diopters base-up in front of one eye and 3 prism diopters base-down in front of the other. The patient now sees two separate images of the eye chart, one from each eye. Ask which image looks clearer. Add +0.25 to the clearer eye to blur it slightly. Ask again. Keep going until both images look equally clear, or until the dominant eye is just slightly clearer than the other.
Once balanced, remove the prisms and reduce the plus equally from both eyes until the patient reaches their best binocular acuity.
Step 9: Verify With the Duochrome Test
The duochrome (red-green) test serves as a final check on your sphere power. The patient views letters displayed on a split background: one side red, one side green. Because red and green light focus at slightly different points inside the eye, this test catches small over- or under-corrections.
If the letters on the red side look sharper, the eye is slightly under-corrected (needs more minus or less plus). If the green side is sharper, you’ve over-corrected. The ideal endpoint is when both sides look equally clear, or when the red side is just barely preferred. If one side is noticeably better, adjust the sphere by 0.25 diopters and test again.
Step 10: Check Near Vision
For patients over 40, or anyone reporting difficulty with close-up tasks, finish by determining a near add. Have the patient read a near card at their typical reading distance (usually about 40 cm). Add plus power binocularly until they can comfortably read the smallest print they need. The near add is recorded separately from the distance prescription.
When Cycloplegic Drops Are Needed
In some patients, the eye’s focusing muscles are too active to get an accurate refraction through fogging alone. Eye drops that temporarily paralyze the focusing muscle are the gold standard for children under 13 or 14, and are also used for patients with crossed eyes, suspected “false” nearsightedness, or hidden farsightedness. Patients between roughly 14 and 35 are often evaluated with a milder drop. The drops take effect within minutes, and the refraction proceeds as normal once accommodation is fully relaxed.
Transposing the Final Prescription
Phoropters typically give results in minus-cylinder format, but some practitioners and labs use plus-cylinder notation. To convert between the two formats, follow three steps:
- Step 1: Add the sphere and cylinder powers together. That sum becomes your new sphere.
- Step 2: Flip the sign of the cylinder (plus becomes minus, minus becomes plus).
- Step 3: Rotate the axis by 90 degrees.
For example, -2.00 +0.50 x 090 becomes -1.50 -0.50 x 180. The two forms describe the exact same correction. Being able to transpose quickly prevents errors when communicating prescriptions across different systems.
Tips for Reliable Results
Room lighting matters. Keep the room dim enough that the chart is the brightest thing in the patient’s visual field, but not so dark that pupils dilate excessively. Present lens choices quickly. If you leave too much time between “option one” and “option two,” the patient’s accommodation shifts and their answer becomes unreliable. When a patient says both options look the same, trust that answer. It usually means you’ve found the correct setting.
Stick to the sequence: objective measurement, fog, sphere, cylinder axis, cylinder power, re-check sphere, balance, verify. Skipping steps or rearranging the order introduces errors that compound as you go. With practice, the whole process becomes fluid, but the order stays the same every time.