A phoropter is operated in a specific sequence: you start by aligning the patient, set a baseline prescription, then refine sphere power, cylinder axis, and cylinder power one eye at a time before balancing both eyes together. The entire process takes practice to perform smoothly, but the underlying logic is straightforward once you understand each step.
Setting Up the Phoropter
Before the patient looks through the device, you need to get the physical setup right. Level the phoropter using the built-in spirit level, which is critical for accurate readings. Adjust the height so the eyepieces sit comfortably in front of the patient’s eyes without them straining or tilting their head.
Set the interpupillary distance (PD) to match the patient’s measurement so each eye looks through the center of its lens aperture. The standard vertex distance, which is the gap between the back of the phoropter lens and the front of the cornea, should be about 13.75 mm when the patient’s cornea is properly aligned with the corneal reference marker on the device. This matters because lens power changes slightly depending on how far it sits from the eye, especially at higher prescriptions.
The patient should be seated 6 meters (20 feet) from the eye chart. Make sure they can see the chart clearly through the phoropter’s open apertures before you start dialing in lenses. Open the projector to show as many lines as possible and ask them to read the smallest line they can identify. This gives you a quick baseline visual acuity and saves time.
Choosing a Starting Point
You don’t refract from zero. You need a reasonable starting prescription to refine from, and there are a few ways to get one. The most common is using the patient’s autorefractor reading, which gives you a machine-generated estimate of their prescription. You can also use their current glasses prescription (measured with a lensometer) or retinoscopy findings. Dial whatever starting values you have into the phoropter’s sphere, cylinder, and axis wheels.
Once the starting point is loaded, occlude the left eye and test the right eye first. Ask the patient to read the smallest line they can see. This confirms your starting point is in the right ballpark before you begin refining.
Refining Sphere Power
Sphere correction addresses nearsightedness or farsightedness. The key principle here is to always start by adding plus power (fogging), which relaxes the eye’s focusing muscles and prevents you from over-correcting with too much minus. A patient whose eye is actively focusing will accept more minus than they actually need, which leads to eye strain later.
Add +0.50 diopters to blur the chart slightly, then reduce in 0.25 diopter steps. At each step, ask the patient if the letters are clearer or more blurred. The goal is to find the least amount of minus (or most amount of plus) that gives the sharpest vision. Make the patient “earn” more minus by requiring them to actually read a smaller line on the chart before you add it. If they can’t see anything smaller, you’ve found the right sphere.
Refining Cylinder Axis
Cylinder correction addresses astigmatism, where the eye focuses light unevenly. Refining astigmatism has two parts: getting the axis (direction) right, then getting the power (strength) right. You do axis first because axis errors blur vision more noticeably than small power errors.
This step uses the Jackson Cross Cylinder (JCC), a flip lens built into the phoropter. To check the axis, align the JCC handle with the current cylinder axis. This automatically positions the JCC’s own axis 45 degrees away from the correcting cylinder. Flip the JCC back and forth and ask the patient which position makes the chart clearer. Rotate the cylinder axis toward the red dot on the JCC when the patient prefers that side. Keep flipping and rotating until the patient reports no difference between the two positions, or until they reverse their preference. At that point, the axis is locked in.
Prepare the patient for this step with clear instructions. Something like: “I’m going to show you two choices. Tell me which one looks clearer, or if they look the same.” Reassure them that “the same” is a perfectly valid answer. Patients often get anxious about giving the “wrong” response, and letting them know there’s no wrong answer produces more reliable results.
Refining Cylinder Power
Once the axis is set, rotate the JCC so its axis lines up parallel with the correcting cylinder’s axis. Now when you flip the lens, you’re testing whether the patient needs more or less cylinder power. If the chart looks clearer with the red dot over the correcting lens, increase the cylinder by 0.25 diopters. If the white (or black) dot side is clearer, decrease by 0.25 diopters. If both sides look the same, the power is correct.
There’s an important compensation rule here: for every 0.50 diopters of cylinder change, you need to adjust the sphere by 0.25 diopters in the opposite direction. This keeps the focal point properly positioned on the retina. So if you increase the cylinder by 0.50, add +0.25 to the sphere. Most clinicians track this as they go rather than correcting at the end.
Repeating for the Other Eye
Once you’ve finished the right eye, occlude it and uncover the left. Run through the same sequence: verify starting visual acuity, refine sphere, refine cylinder axis, refine cylinder power. Each eye is treated independently during this monocular phase.
Binocular Balancing
After both eyes are individually refracted, you need to balance them so neither eye is over-corrected relative to the other. The goal is to equalize the accommodative effort between the two eyes. Several methods exist, but the general approach involves uncovering both eyes, slightly fogging both with plus power so acuity drops to a moderate line, and then alternately adjusting each eye’s sphere until the patient reports equal clarity from both sides.
One common technique uses prisms built into the phoropter to vertically separate the image so each eye sees the same line of letters at a different height. You then adjust sphere power on each side until the patient reports both lines are equally clear (or equally blurry). Once balanced, you reduce plus power equally from both eyes until the patient reaches their best acuity.
Near Vision and Presbyopia Testing
For patients over 40, or anyone reporting difficulty reading up close, you’ll test near vision after completing the distance refraction. Attach the near point card to the phoropter’s near point rod and position it at 40 cm (about 16 inches). The patient views the card through their distance correction.
If they can’t read comfortably at that distance, you add plus sphere in front of both eyes until near text becomes clear. This additional plus power is the “near add.” For the Borish Vectographic card, the patient compares the boldness of vertical versus horizontal lines on a grid. If horizontal lines appear more prominent, that signals the eye isn’t focusing well up close, and you increase plus power until both directions look equally distinct.
You can also measure how much focusing ability the patient has left. With the patient reading the smallest visible line on the near card, slowly move the card closer until the print blurs. The distance at which blur occurs tells you the amplitude of accommodation, which helps determine how much near add they need.
Digital vs. Manual Phoropters
Manual phoropters use physical dials and knobs to rotate lenses into position. You turn the sphere dial, click the cylinder wheel, and physically flip the JCC. Digital phoropters use motorized lens assemblies controlled through a tablet or console, which lets you change lenses with a tap rather than a turn. The clinical logic is identical; only the interface changes.
Digital systems offer a few practical advantages. They can import autorefractor data directly, eliminating transcription errors from manually reading and dialing in numbers. The examiner can also sit farther from the patient, which became more relevant during infectious disease concerns. Some newer digital systems keep both eyes partially open during monocular testing, allowing the brain to maintain fusion. In a traditional phoropter, the occluded eye sees nothing, which can slightly alter the refraction compared to real-world viewing.
Cleaning and Maintenance
The forehead rest, chin rest, and any knobs the patient touches should be disinfected between every patient using alcohol wipes or surgical spirit. The internal lenses are harder to clean once exposed to contaminants, which is one reason some clinics shifted away from phoropters toward trial frames during infection control surges.
Haag-Streit, a major phoropter manufacturer, recommends annual inspection by a service technician for function and safety checks. Between professional servicing, regularly verify that the phoropter is level using its spirit level, especially if the mounting arm gets bumped. Digital models run a self-test on startup and flag calibration errors, but you should still confirm that displayed values match what you’d expect from known test lenses.