Perform Either Step 3a or Steps 3b and 3c

Step 3a. Use prior information to obtain initial estimate of refractive error

• Measure the patient’s current eyeglasses, if available, and dial these values into the phoropter. (Use of the lensmeter to measure—“neutralize”—glasses is described in Chapter 8.) Use the result of a previous refraction, such as a previous prescription for glasses as provided by the patient, or as recorded in the patient’s medical record.

• Perform retinoscopy (see Chapter 4).

  • If lenses in the phoropter are clean, retinoscopy may be performed through the phoropter, so that the results of the retinoscopy will be immediately available as the phoropter settings to serve as the starting point for subjective refraction. It is a good habit to write down the retinoscopic findings for comparison with the final refraction.

• Use an autorefractor or a photorefractor.

  • This procedure seldom saves time if you have to take the patient to a different room. Autorefractors are essentially automated retinoscopes. They work well on eyes with clear media and adequate pupils in patients with steady, central fixation. In patients with media opacities, small pupils, or an inability to sit still—just the patients with whom you could appreciate some automated help—they generally fail.

• If there is no known astigmatic correction, proceed directly to step 5; if there is an astigmatic correction, go to step 4.

Step 3b. Make direct initial measurement of spherical refractive error

• If no preliminary estimate is available from old glasses, the medical record, retinoscopy or autorefraction, determine the best spherical correction by offering the patient successive choices of alternative spherical corrections. Starting with the spherical power at 0.00, use the low-power sphere lens wheel on the phoropter (or hand-held lenses if using a trial frame), offer the patient a choice between a spherical lens power of +0.25 D or –0.25 D. This gives the patient a choice between spherical lenses that differ by 0.50 D. (If the patient’s visual acuity is very poor, offer a greater difference between the 2 choices of sphere power.) Use the language “Which is better, 1 or 2?” This “forced choice” is referred to as a binary comparison.

  • Make certain the patient is referring to your choice between lens 1 or lens 2, not line 1 or line 2 of the eye chart. If necessary, mask off all but 1 line of the chart.

  • If the patient’s acuity is very poor, it may be preferable to offer binary comparisons with choices that differ by 1.00 D.

• Advance the sphere setting 0.25 D toward the preferred alternative, and then offer the next binary comparison to bracket the patient’s most recent choice of sphere power setting, again with a 0.50 D difference. (For example, if the patient’s initial preference is for the –0.25 lens, the next choice offered should be between 0.00 and –0.50 D lenses.)

• Repeat this process until the patient suggests that the 2 choices are about the same.

• Recheck this endpoint, using choices that differ by only 0.25 D.

• Generally, try to move most of the time from more plus to less plus (or from less minus to more minus). By offering the more plus or less minus choice first, you will stimulate accommodation only minimally.

  • Try to be sensitive to the patient’s responses. As you near the endpoint, responses are typically slower. The patient may simply stop responding rather than volunteer that the choices are about the same. This is still useful information.

• Do not worry about making a mistake by going too far—the method is self-correcting.

• Once the best possible spherical correction has been determined, go to step 3c to determine if there is any astigmatic refractive error.

Step 3c. Detect astigmatism

Now that you have found the optimal correction by using only spheres, determine whether there is any astigmatism as follows.

(Directions for use with a minus cylinder phoropter)

1. Dial in –0.50 D of the cylinder.

2. Increase the sphere power by +0.25 D.

3. Rotate the cylinder axis knob slowly once or twice around, and ask the patient to tell you to stop when he or she perceives the clearest vision. (Alternatively, you may ask the patient to turn the knob and leave it where his or her vision is best.)

4. If the patient reports that rotating the dial makes no difference to the visual clarity—even if the vision is rather poor—then you may assume that the patient has no significant astigmatism, and proceed to step 5. If the patient can select a preferred axis, you have detected an astigmatic refractive error. Leave the cylinder dial at the preferred axis orientation, and proceed to step 4.

Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.