Corboy JM. The Retinoscopy Book: An Introductory Manual for Eye Care Professionals. 4th ed. Thorofare, NJ: Slack; 1995. Wirtschafter JD, Schwartz GS. Retinoscopy. In: Tasman W, Jaeger EA, eds. Duane’s Clinical Ophthalmology [CD-ROM]. Vol 1.

Philadelphia: Lippincott Williams & Wilkins; 2006:chap 37.

Subjective Refraction Techniques

In subjective refraction techniques, the examiner relies on the patient’s responses to determine the refractive correction. If all refractive errors were spherical, subjective refraction would be easy. However, determining the astigmatic portion of the correction is more complex, and various subjective refraction techniques may be used. The Jackson cross cylinder is the most common instrument used in determining the astigmatic correction. However, we begin by discussing the astigmatic dial technique because it is easier to understand.

Astigmatic Dial Technique

An astigmatic dial is a test chart with radially arranged lines that may be used to determine the axes of astigmatism. A pencil of light from a point source is imaged by an astigmatic eye as a conoid of Sturm. The spokes of the astigmatic dial that are parallel to the principal meridians of the eye’s astigmatism are imaged as sharp lines, which correspond to the focal lines of the conoid of Sturm.

Figure 3-15A shows an eye with compound hyperopic astigmatism and how it sees an astigmatic dial. The vertical line of the astigmatic dial is the blackest and sharpest because the vertical focal line of each conoid of Sturm is closer to the retina than the horizontal focal line is. By accommodating, however, the patient might pull both focal lines forward, far enough to make even the horizontal line of the astigmatic dial clear. To avoid accommodation, fogging is used. Sufficient plus sphere is placed before the eye to pull both focal lines into the vitreous, creating compound myopic astigmatism (Fig 3-15B).

Figure 3-15 Astigmatic dial technique. A, Conoid of Sturm and retinal image of an astigmatic dial as viewed by an eye with compound hyperopic astigmatism. B, Fogging to produce compound myopic astigmatism. C, The conoid of Sturm is collapsed to a single point. D, Minus sphere is added (or plus sphere subtracted) to produce a sharp image, and a visual

acuity chart is used for viewing.

Because accommodating with the eye fogged causes increased blurring of the lines, the patient relaxes accommodation. The focal line closest to the retina can then be identified with certainty as the horizontal line because it is now the blackest and sharpest line of the astigmatic dial. Note that the terms blackest and sharpest are more easily understood by patients and should be used in place of the word clearest.

After the examiner locates 1 of the principal meridians of the astigmatism, the conoid of Sturm can be collapsed by moving the anterior focal line back toward the posterior focal line. This task can be accomplished by adding a minus cylinder with an axis parallel to the anterior focal line. In Figure 3-15C the vertical focal line has been moved back to the position of the horizontal focal line and collapsed to a point by the addition of a minus cylinder with an axis at 90°. Notice that the minus cylinder is placed with its axis perpendicular to the blackest meridian on the astigmatic dial. Also note that as the conoid of Sturm is collapsed, the focal lines disappear into a point focus.

All of the lines of the astigmatic dial now appear equally black but still are not in perfect focus, because the eye remains slightly fogged to control accommodation. At this point, a visual acuity chart is used; plus sphere is removed until the best visual acuity is obtained (Fig 3-15D).

In summary, the following steps are used in astigmatic dial refraction:

1.Obtain the best visual acuity using spheres only.

2.Fog the eye to approximately 20/50 by adding plus sphere.

3.Ask the patient to identify the blackest and sharpest line of the astigmatic dial.

4.Add minus cylinder with the axis perpendicular to the blackest and sharpest line until all lines appear equal. (If using a positive cylinder phoropter, add plus cylinder with the axis parallel to the blackest and sharpest line until all lines appear equal.)

5.Reduce plus sphere (or add minus) until the best visual acuity is obtained with the visual acuity chart.

Astigmatic dial refraction can also be performed with plus cylinder equipment, but this technique must be used in a way that simulates minus cylinder effect. All of the above steps remain the same except for step 4, which becomes “Add plus cylinder with the axis parallel to the blackest and sharpest line.” As each 0.25 D of plus cylinder power is added, change the sphere simultaneously by 0.25 D in the minus direction. Doing so simulates minus cylinder effect exactly by moving the anterior focal line posteriorly without changing the position of the posterior focal line.

Michaels DD. Visual Optics and Refraction: A Clinical Approach. 3rd ed. St Louis: Mosby; 1985:319–322.

Stenopeic Slit Technique

The stenopeic slit is an opaque trial lens with an oblong slit whose width forms a pinhole with respect to vergence perpendicular to the slit (Fig 3-16). If an examiner is unable to decipher the astigmatism by performing the usual retinoscopy because of the subject eye’s irregular astigmatism or unclear media, he or she may neutralize the refractive error with spherical lenses and the slit at various meridians to find a spherocylindrical correction. This correction can then be refined subjectively. This process is especially useful for patients with small pupils and lenticular or corneal opacities. If the subject can accommodate, fog and unfog using plus sphere to find the most plus power accepted. Then turn the slit until the subject says the image is sharpest. If, for example, –3.00

D sphere is best there, when the slit is oriented vertically, this finding indicates –3.00 D at 90° in a power cross. If the best sphere with the slit oriented horizontally is –5.00 D, then the result is –3.00 – 2.00 × 90.

Figure 3-16 Stenopeic slit. The image on the right demonstrates the placement of a spherical lens in front of the stenopeic slit in order to determine the best visual acuity. (Courtesy of Tommy Korn, MD.)

Cross-Cylinder Technique

The Jackson cross cylinder, in Edward Jackson’s words, is probably “far more useful, and far more used” than any other lens in clinical refraction. Every ophthalmologist should be familiar with the principles involved in its use. Although the cross cylinder is usually used to refine the cylinder axis and power of a refraction that has already been obtained, it can also be used for the entire astigmatic refraction.

The first step in cross-cylinder refraction is adjusting the sphere to yield the best visual acuity with accommodation relaxed. Begin by placing the prescription the patient is wearing into a trial frame or phoropter. Fog the eye to be examined with plus sphere while the patient views a visual acuity chart; then decrease the fog until the best visual acuity is obtained. If astigmatism is present, decreasing the fog places the circle of least confusion on the retina, creating a mixed astigmatism. Then use test figures that are 1–2 lines larger than the patient’s best visual acuity. At this point, introduce the cross cylinder, first for refinement of cylinder axis and then for refinement of cylinder power.

If no cylindrical correction is present initially, the cross cylinder may still be used, placed at 90° and 180°, to check for the presence of astigmatism. If a preferred flip position is found, cylinder is added with the axis parallel to the respective plus or minus axis of the cross cylinder until the 2 flip choices are equal. If no preference is found with the cross-cylinder axes at 90° and 180°, then check the axes at 45° and 135° before assuming that no astigmatism is present. Once any cylinder power is found, axis and power should be refined in the usual manner.

Another method of determining the presence of astigmatism is to dial 0.50 D of cylinder into the

phoropter (while preserving the spherical equivalent with a compensatory 0.25 D change in the sphere). Ask the patient to slowly rotate the cylinder axis once around using the knob on the phoropter. If doing so has no effect, there is no clinically significant astigmatism. If the patient finds a preferred position, it becomes the starting point for the cross-cylinder refinement.

Always refine cylinder axis before refining cylinder power. This sequence is necessary because the correct axis may be found in the presence of an incorrect power, but the full cylinder power is found only in the presence of the correct axis.

Refinement of cylinder axis involves the combination of cylinders at oblique axes. When the axis of the correcting cylinder is not aligned with that of the astigmatic eye’s cylinder, the combined cylinders produce residual astigmatism with a meridian roughly 45° away from the principal meridians of the 2 cylinders. To refine the axis, position the principal meridians of the cross cylinder 45° away from those of the correcting cylinder (if using a handheld Jackson cross cylinder, the stem of the lens handle will be parallel to the axis of the correcting cylinder). Present the patient with alternative flip choices, and select the choice that is the blackest and sharpest to the patient. Then rotate the axis of the correcting cylinder toward the corresponding plus or minus axis of the cross cylinder (plus cylinder axis is rotated toward the plus cylinder axis of the cross cylinder, and minus cylinder axis is rotated toward the minus cylinder axis of the cross cylinder). Low-power cylinders are rotated in increments of 15°; high-power cylinders are rotated by smaller amounts, usually 5°. Repeat this procedure until the flip choices appear equal.

To refine cylinder power, align the cross-cylinder axes with the principal meridians of the correcting lens (Fig 3-17). The examiner should change cylinder power according to the patient’s responses; the spherical equivalent of the refractive correction should remain constant to keep the circle of least confusion on the retina. Ensure that the correction remains constant by changing the sphere half as much and in the opposite direction as the cylinder power is changed. In other words, for every 0.50 D of cylinder power change, change the sphere by 0.25 D in the opposite direction. Periodically, the sphere power should be adjusted for the best visual acuity.