What is the phoropter and what is its purpose?

The phoropter is an instrument containing an array of lenses that can be quickly dialed into the patient's line of sight during subjective refraction. It allows the examiner to precisely adjust sphere, cylinder, axis, prism, and other parameters while the patient compares options. The physician uses it to refine the objective refraction result into the final best-corrected prescription.

How should a CPO prepare a patient for subjective refraction?

Explain that the physician will present two lens choices and ask which is clearer. Reassure the patient there are no right or wrong answers and that 'about the same' is a valid response. Tell them to respond based on overall clarity rather than edge sharpness, and to avoid squinting. Also set the phoropter to the autorefraction starting point and ensure the PD and vertex distance are correctly adjusted before the physician begins.

What is vertex distance and why does it matter?

Vertex distance is the space between the back of the phoropter lens (or spectacle lens) and the front of the cornea, standardly 12-14 mm. For prescriptions above ±4.00 D, even a few millimeters of change in this distance alters the effective power of the lens significantly. Accurate, consistent vertex distance positioning ensures the phoropter result translates correctly into the spectacle prescription.

Subjective Refraction Assistance: CPO Exam Guide

The Role of the CPO in Refraction

Subjective refraction is the process of fine-tuning the optical correction by asking patients to compare lens choices. While the physician or optometrist performs the actual refraction, as a CPO you play a key supporting role: preparing the patient and equipment, setting up the phoropter from the objective refraction result, and ensuring the patient understands how to respond during the exam.

The Phoropter

The phoropter (also called a refractor head) is the instrument that holds an array of lenses and allows the examiner to quickly switch between different powers and combinations during refraction. Key components include:

Sphere dials: adjusts the plus or minus power in 0.25 D increments
Cylinder dials: adjusts astigmatism correction
Axis wheel: rotates the cylinder axis
Occluder: blocks one eye for monocular testing
Cross cylinder (Jackson Cross Cylinder, JCC): refines cylinder power and axis
Prism: for binocular testing and balance
Maddox rod, ±0.50 flipper, auxiliary lenses: for special tests

Before the physician enters the room, set the phoropter to the autorefractor result (or the patient's current glasses prescription if no autorefraction was done). This saves several minutes per patient and keeps the clinic flowing efficiently.

Preparing the Patient

Patient cooperation during refraction directly affects the quality of the result. Good preparation includes:

Explain that the physician will show two choices and ask which is clearer: "Option 1 or option 2?" or "Which is better, A or B?"
Reassure the patient that there is no right or wrong answer — they should simply say what looks clearer to them
Advise them to respond based on the clarity of the whole image, not just the sharpness of edges
Tell them it is fine to say "about the same" or "I can't tell" if they genuinely cannot distinguish a difference
Instruct them to avoid squinting, as squinting creates a pinhole effect that alters results

One of the most common patient errors during refraction is trying to "help" by always picking one option even when they cannot tell a difference. This leads to over-minusing. Emphasizing that "about the same is a perfectly valid answer" reduces this problem.

Vertex Distance

The vertex distance is the distance between the back of the phoropter lens and the front of the patient's cornea. Standard vertex distance is 12-14 mm. Adjusting the forehead rest and chin rest to position the patient correctly ensures the refraction translates accurately to the spectacle prescription.

For prescriptions above ±4.00 D, a vertex distance change significantly affects effective lens power, making consistent positioning important.

Setting PD in the Phoropter

The phoropter must be set to the patient's pupillary distance (PD) so the optical centers align with the visual axes. Most phoropters have a PD adjustment knob or dial. The monocular PD for each eye should be set individually for accurate results.

Setting the phoropter PD to the binocular PD split evenly rather than to the patient's individual monocular PD values. A patient with 32/30 monocular PD (right/left) needs asymmetric settings, not 31/31.

After the Refraction

Once refraction is complete, you may be asked to:

Record the final refraction values
Check the best-corrected visual acuity (BCVA) with the new refraction dialed in
Verify the result against the current glasses prescription to assess the change

Key Takeaways

Set the phoropter to the objective refraction result before the physician enters
Coach patients to answer honestly: "about the same" is a valid answer
Vertex distance must be set correctly, especially for prescriptions above ±4.00 D
Set monocular PD values individually in the phoropter
Record BCVA with the final refraction dialed in