Unit 4: Subjective Refraction | Visual Optics-II | 4th Semester of Bachelor of Optometry

Subjective Refraction

Subjective refraction is the process of refining the patient’s prescription by relying on their visual responses, typically asking which lens combination provides the clearest and most comfortable vision. While objective methods such as retinoscopy provide an accurate baseline, subjective refraction allows for personalized correction that optimizes acuity, comfort, and binocular balance. This unit explores the principles, methods, and clinical significance of subjective refraction.

Principle of Subjective Refraction

The principle of subjective refraction is based on maximizing visual acuity by adjusting spherical and cylindrical lenses according to patient responses. The patient’s perceptual judgment of clarity is used to refine the optical correction until the sharpest, most comfortable vision is achieved. It also ensures proper binocular vision and balance, which objective refraction alone cannot guarantee.

Steps in Subjective Refraction

1. Starting point: Use retinoscopy or autorefractor results as the initial estimate.
2. Sphere refinement: Use the fogging technique to control accommodation and refine spherical correction.
3. Cylinder determination: Employ astigmatic dials, fan and block test, and Jackson’s Cross Cylinder (JCC) to determine axis and power of astigmatism.
4. Duochrome test: Use chromatic aberration principles to refine spherical endpoint.
5. Binocular balancing: Equalize accommodative stimulus between both eyes using alternate occlusion, prism dissociation, or fogging balance techniques.
6. Binocular refraction: Final refinement under binocular conditions to ensure comfortable vision and fusion.

Fogging Principle

Fogging is a technique used to relax accommodation during subjective refraction. A plus lens is introduced, shifting the focus in front of the retina (fogging the image). The plus power is then gradually reduced until the clearest vision is achieved at the far point without stimulating accommodation. This ensures accurate spherical correction and prevents over-minusing.

Astigmatism Tests

• Clock Dial Test: Uses radial lines to identify the meridian of astigmatism. Lines appearing clearest indicate the closest focus meridian.
• Fan and Block Test: A combination of fixed and rotating dials for refining both axis and power of astigmatism.
• Jackson’s Cross Cylinder (JCC): A powerful tool for precise determination of cylinder axis and power. The examiner flips the JCC lens, and the patient reports which orientation appears clearer.

Duochrome Test

The duochrome test exploits chromatic aberration of the eye. With equal spherical correction, letters on red and green backgrounds should appear equally clear. If letters on the red side appear clearer, the patient is under-corrected (needs more minus/less plus). If green letters are clearer, the patient is over-corrected (needs more plus/less minus).

Binocular Balancing Techniques

Binocular balancing ensures that both eyes accommodate equally, avoiding unequal strain. Common methods include:

• Alternate Occlusion: Alternately cover each eye, equalizing clarity at the best spherical correction.
• Prism Dissociation: Use vertical prisms to dissociate the eyes and balance clarity between them.
• Dissociated Duochrome Balance: Combines duochrome principles with binocular dissociation.
• Borish’s Dissociated Fogging: One eye is fogged while the other refines; then balance is achieved by alternating.

Binocular Refraction

After monocular refinement and balancing, refraction is completed binocularly to simulate real-life viewing conditions. This step fine-tunes the prescription under normal binocular fusion, ensuring both clarity and comfort.

Clinical Importance

• Provides the most accurate and comfortable prescription tailored to patient perception.
• Essential for correcting astigmatism precisely.
• Ensures binocular visual balance, reducing symptoms of asthenopia, diplopia, and fatigue.
• Prevents over-minusing, which can lead to accommodative strain and progression of myopia in young patients.
• Critical in professions requiring high visual performance, such as pilots and athletes.

Limitations

• Requires patient cooperation and reliable responses.
• Less effective in children, non-verbal patients, or those with cognitive impairment.
• Can be influenced by patient’s subjective biases and fatigue.

Fixed Astigmatic Dial, Fan and Block Test, and Jackson’s Cross Cylinder (JCC)

Accurate detection and correction of astigmatism is a vital step in subjective refraction. Several clinical tools and techniques are available to refine the axis and power of cylindrical correction. Among them, the astigmatic dial, the fan and block test, and the Jackson’s Cross Cylinder (JCC) test are the most commonly used. Each test has its own principles, procedures, and clinical significance. Together, they allow precise refinement of cylinder prescriptions to achieve the sharpest and most comfortable vision.

1. Fixed Astigmatic Dial (Clock Dial)

Principle

The fixed astigmatic dial, often called the clock dial, is based on the optical principle that in uncorrected astigmatism, lines oriented closest to the axis of the correcting cylinder appear clearer than those perpendicular to it. By asking patients to identify which radial lines appear clearest, the examiner estimates the axis of astigmatism.

Procedure

1. A clock dial chart with 12 or 24 radial lines (like a clock face) is projected at 6 meters.
2. The patient is fogged with plus lenses to control accommodation.
3. The patient is asked which line(s) appear darkest or clearest.
4. The axis of the correcting cylinder corresponds to the clearest line direction.
5. Cylinder power is refined by adjusting until all lines appear equally dark and clear.

Clinical Use

• Gives an approximate axis of astigmatism, later refined with JCC.
• Useful as a starting point for patients with newly detected astigmatism.

2. Fan and Block Test

Principle

The fan and block test is a modification of the astigmatic dial that combines radial lines (fan) and rectangular blocks to refine both axis and cylinder power. It reduces subjective confusion and provides better control over axis determination.

Procedure

1. The fan chart, containing lines radiating out at various meridians, is presented to the patient.
2. The patient is asked to identify the clearest line or group of lines.
3. The examiner adjusts cylinder axis until all lines appear equally clear.
4. The block portion of the test provides rectangular sections that appear more or less sharp depending on residual astigmatism, allowing cylinder power refinement.

Clinical Use

• Refines axis more accurately than the clock dial alone.
• Combines axis and power refinement into a single test.
• Often used after retinoscopy or autorefractor results as a subjective verification tool.

3. Jackson’s Cross Cylinder (JCC)

Principle

The JCC test is the most precise and widely used technique for refining cylinder axis and power. A JCC lens is a small hand-held lens with equal plus and minus cylinder powers at perpendicular meridians (e.g., ±0.25 DC). When flipped, it alternately makes one meridian clearer than the other, allowing fine-tuning of cylinder correction.

Procedure for Axis Refinement

1. Place the JCC lens over the patient’s correction, centered at the suspected axis.
2. Flip the lens, asking the patient which orientation appears clearer.
3. Rotate the axis of the cylinder toward the clearer position.
4. Repeat until the patient reports no difference between orientations (final axis).

Procedure for Power Refinement

1. Place JCC at the finalized axis, with handles aligned.
2. Flip the lens, asking which option is clearer.
3. If option with more minus cylinder is clearer → increase cylinder power.
4. If option with less minus is clearer → decrease cylinder power.
5. Recheck axis after power adjustment.

Clinical Use

• Provides the most precise subjective refinement of astigmatism.
• Used routinely in all comprehensive refractions after retinoscopy.
• Particularly useful in fine-tuning small amounts of cylinder (<1.00 D).

Comparison of Techniques

Technique	Purpose	Accuracy	Clinical Use
Clock Dial	Initial axis estimation	Moderate	Screening, starting point for astigmatism refinement
Fan and Block	Axis and power refinement	High	More advanced than clock dial, less precise than JCC
Jackson’s Cross Cylinder	Precise axis and power refinement	Very high	Final confirmation and prescription

Duochrome Test

The duochrome test, also called the red-green test, is a subjective refraction technique used to refine the spherical component of a prescription. It is based on the principle of longitudinal chromatic aberration of the human eye, which causes light of different wavelengths (colors) to focus at slightly different points along the visual axis. The duochrome test is quick, easy to perform, and provides an accurate method of confirming the endpoint of spherical refraction.

Optical Principle

The human eye exhibits about 1.25–2.00 D of longitudinal chromatic aberration between the red (~620–640 nm) and green (~535–545 nm) ends of the visible spectrum:

• Red light (longer wavelength) focuses behind green light.
• Green light (shorter wavelength) focuses in front of red light.

A duochrome chart presents optotypes (letters or symbols) on split red and green backgrounds. Depending on the patient’s refractive status:

• If letters on the green side are clearer → image focus is in front of the retina → the eye is over-plussed (or under-minused).
• If letters on the red side are clearer → image focus is behind the retina → the eye is over-minused (or under-plussed).
• If letters are equally clear on both sides → circle of least confusion lies on the retina → spherical correction is optimal.

Procedure

1. Place the duochrome chart at standard testing distance (6 m or 20 ft for distance vision).
2. Present the chart binocularly (or monocularly in some cases).
3. Ask the patient to compare clarity of letters on the red vs. green side.
4. Adjust spherical correction as follows:
   • If red side clearer → add plus (or reduce minus).
   • If green side clearer → add minus (or reduce plus).
   • Continue until both sides are equally clear, or the patient reports no difference.

Clinical Applications

• Final endpoint of sphere: Confirms the most accurate spherical correction after subjective refinement.
• Control of accommodation: Ensures patient is not over-minused, especially in young individuals with active accommodation.
• Binocular balancing: Can be used dissociated with prisms to balance accommodation between eyes.
• Near refraction: A duochrome chart can also be used at 40 cm for refining near prescriptions.

Interpretation of Results

Equally clear letters = circle of least confusion on retina → endpoint. Red clearer = over-minused or under-plussed → add plus. Green clearer = over-plussed or under-minused → add minus.

Advantages

• Simple, quick, and requires minimal explanation to the patient.
• Provides objective confirmation of spherical endpoint.
• Useful for controlling overcorrection in myopia and latent hyperopia.

Limitations

• Requires good visual acuity (usually ≥6/9 or 20/30); not useful in patients with reduced vision.
• Not reliable in color vision deficiencies (particularly red-green anomalies).
• Less effective in older patients with small pupils and reduced sensitivity to chromatic differences.
• Ambient lighting and projector quality may affect accuracy.

Clinical Example

A 25-year-old patient undergoing subjective refraction reports that letters on the red side of the duochrome chart appear clearer. The examiner reduces minus by 0.25 D, after which the patient reports equal clarity on both sides. This confirms the final spherical endpoint, preventing over-minusing.

Binocular Balancing and Binocular Refraction

After monocular subjective refraction has been performed for each eye separately, it is essential to ensure that both eyes are working together efficiently and comfortably under binocular conditions. This process involves two critical steps: binocular balancing and binocular refraction. These procedures optimize the prescription not just for maximum acuity in each eye, but also for coordinated binocular vision, reducing symptoms of asthenopia, diplopia, or accommodative strain.

Binocular Balancing

Definition: Binocular balancing is the process of equalizing accommodative stimulus and response between the two eyes after monocular refraction. Its primary purpose is to ensure that both eyes contribute equally to vision and that one eye is not over-accommodating relative to the other.

Objectives of Binocular Balancing

• Ensure equal accommodative demand in both eyes.
• Prevent one eye from being over-minused or over-plussed.
• Improve comfort for sustained near and distance tasks.
• Reduce binocular stress that could lead to asthenopia, headaches, or suppression.

Techniques of Binocular Balancing

Several clinical methods are used, each with its own advantages:

1. Alternate Occlusion Method

• Performed after both eyes are corrected to approximate acuity.
• Each eye is alternately occluded while the patient looks at a letter chart just above threshold level.
• If one eye consistently reports clearer letters, adjust the lens power slightly until clarity is equal in both eyes.
• Advantage: Simple and quick.
• Limitation: Does not dissociate completely; may allow some fusion cues.

2. Prism Dissociation Method

• Vertical prisms (usually 3Δ base-up before one eye and 3Δ base-down before the other) are placed in front of the eyes.
• This dissociates the retinal images vertically, so fusion is impossible.
• The patient sees two lines or sets of letters; plus or minus lenses are adjusted until both appear equally clear.
• Advantage: Precise dissociation; reduces suppression.
• Limitation: Some patients may find diplopia uncomfortable.

3. Dissociated Duochrome Balance

• A duochrome chart is used with vertical prism dissociation.
• Each eye sees letters on red and green backgrounds separately.
• Lenses are adjusted until both eyes perceive equal clarity on both colors.
• Advantage: Combines chromatic aberration control with dissociation.
• Limitation: Requires good visual acuity and color perception.

4. Borish Dissociated Fogging Technique

• Both eyes are fogged with additional plus lenses.
• One eye is then defogged while the other remains fogged, so that only one eye provides a clear image.
• Each eye is refined alternately under binocular conditions until clarity is equalized.
• Advantage: Maintains binocular conditions while dissociating accommodative demand.
• Limitation: Slightly more time-consuming than other methods.

Binocular Refraction

Binocular refraction is the final step of subjective refraction, performed under normal binocular viewing conditions (both eyes open and fused). It ensures that the prescription is accurate, comfortable, and effective when the eyes are used together in daily life.

Techniques of Binocular Refraction

• Binocular Sphere Refinement: After balancing, the patient views the chart with both eyes open. Sphere is fine-tuned binocularly, usually using fogging or duochrome.
• Binocular Cross-Cylinder: In some cases, Jackson’s Cross Cylinder (JCC) is used binocularly to refine residual astigmatism with fusion maintained.
• Binocular Fogging Technique: Similar to Borish’s balancing, both eyes are slightly fogged and defogged simultaneously under binocular conditions to achieve the final prescription.

Clinical Considerations

• Important for patients with binocular vision anomalies, as monocular findings may not align with binocular comfort.
• Ensures optimal prescription for tasks such as reading, computer use, and distance vision in real-life conditions.
• Particularly useful for patients with high accommodative-convergence interactions, like esophoria or exophoria.

Summary

Binocular balancing and binocular refraction are crucial steps in subjective refraction. Balancing ensures equal accommodative demand between both eyes through methods like alternate occlusion, prism dissociation, dissociated duochrome, and Borish’s technique. Binocular refraction then fine-tunes the final prescription under fused conditions, ensuring clarity, comfort, and binocular harmony. Together, they transform monocular findings into a prescription that supports stable, efficient, and comfortable binocular vision in everyday life.

For more units of "Visual Optics-II" click below on text 👇 

✅ Unit 1

✅ Unit 2

✅ Unit 3

✅ Unit 5