The Jackson cross cylinder (JCC) is the standard tool for refining the cylinder axis and power during subjective refraction. Designed by Edward Jackson in the 1880s, it presents the patient with two equal-and-opposite cylinder choices in rapid succession, allowing the examiner to bracket the correct axis and power without resetting the phoropter between trials. ABO-NCLE, IJCAHPO COA and COT, and paraoptometric exams all expect candidates to understand the JCC, its proper orientation, and the axis-then-power sequence.
Concept Overview
A Jackson cross cylinder is a lens with equal plus and minus cylinder powers placed at right angles. The most common phoropter-mounted JCC is +/-0.25 D, with +/-0.50 D handheld units used for low-vision or low-acuity patients. The lens has a handle that lies 45 degrees away from each cylinder axis. White and red dots on the rim indicate the plus and minus cylinder axes respectively (white for plus, red for minus, by ANSI convention).
When the examiner flips the JCC, the plus and minus axes swap positions. The patient compares the two views and chooses the clearer one. The examiner then makes a small change in axis or power and repeats until both flip positions look equally blurred, indicating that the cylinder is correct.
The Spherical Equivalent Principle
The JCC has a spherical equivalent of zero. Adding +0.25 cylinder along one meridian and -0.25 along the other creates no net change in mean spherical power. This is essential because it lets the examiner change cylinder without disturbing best sphere. However, when the cylinder power is changed during refinement, the operator must compensate sphere by half the cylinder change in the opposite direction to keep the circle of least confusion on the retina. A change of -0.50 D cylinder requires +0.25 D added to sphere.
How It's Performed
JCC refinement comes after a starting cylinder estimate from retinoscopy, autorefraction, or the patient's old prescription. The patient should already have a best sphere and approximate cylinder dialed into the phoropter. The standard sequence is axis first, then power.
- Reduce visual acuity to 20/30 or 20/40. Refining JCC at 20/20 makes the differences too small to perceive. Show a single line near threshold.
- Refine axis first. Position the JCC handle parallel to the current cylinder axis. The plus and minus axes of the JCC now straddle the cylinder axis at 45 degrees on either side. Show the patient view 1, flip, show view 2, and ask "which is clearer, one or two, or are they the same?"
- Rotate the cylinder axis toward the red dot of the preferred view if minus cylinder is being refined. The standard rule: chase the red. Rotate in 5 degree steps initially, then 2 to 3 degrees as you converge.
- Repeat until the patient reports the two views are equally blurred. That is the correct axis. Move on to power.
- Refine power. Position the JCC so its axes align with the cylinder axis (handle now at 45 degrees to the cylinder axis). Flip and ask which view is clearer.
- Add cylinder in the direction of the preferred view. If the patient prefers the view with the JCC minus axis aligned with the existing minus cylinder axis, add minus cylinder in 0.25 D steps. If they prefer plus aligned, reduce minus cylinder.
- Compensate sphere. For every 0.50 D added to cylinder, add 0.25 D in the opposite sign to sphere. This keeps the circle of least confusion at the retina and preserves best acuity.
- Stop when the two flip views appear equally blurred. That is the correct power.
- Recheck axis if power changed substantially. Axis can shift slightly as power is refined.
- Recheck best sphere with a +/-0.25 sphere flip or duochrome test before declaring the refraction final.
Refining Cylinder When None Is in the Phoropter
If retinoscopy shows no cylinder but the patient's acuity is below 20/20 with best sphere, you can use the JCC to check for masked cylinder. Place a small trial cylinder (commonly -0.50 D at 90 or 180 degrees) and run JCC. If the patient consistently rejects the trial cylinder in both axis positions, no cylinder is needed. If they accept and a clear axis emerges, refine and document.
Equipment
Most refractions use the JCC built into the phoropter. The Reichert and Topcon phoropters mount a +/-0.25 D JCC on a swing-arm, with a thumbwheel that rotates the JCC to align with the current cylinder axis automatically. A flip lever swaps positions. Handheld JCCs come in +/-0.25, +/-0.50, and occasionally +/-1.00 D versions for low-vision use, where larger cylinder steps make the difference perceptible.
Trial-frame refraction uses a handheld JCC. The examiner must rotate it manually so the handle either parallels (axis check) or is 45 degrees to (power check) the trial cylinder axis. The white dot marks the plus cylinder axis and the red dot marks the minus cylinder axis, per ANSI Z80 standards. Most clinicians refract in minus cylinder, so they "chase the red" during axis refinement.
Choice of JCC Power
- +/-0.25 D: standard for routine refraction at 20/30 or better.
- +/-0.50 D: for patients whose best corrected acuity is 20/40 to 20/80, or who cannot reliably distinguish 0.25 D differences.
- +/-1.00 D: low-vision refraction at 20/100 or worse. Common in low-vision clinics.
Interpretation
The end point of JCC refinement is patient indifference. When the patient genuinely cannot pick a preferred view across multiple flips, you have reached the correct axis (during axis refinement) or correct power (during power refinement). Forced choice is acceptable on the first few flips while you bracket, but the final answer is "they look the same."
Common Errors and Pitfalls
- Refining at 20/20 acuity: differences are too small for the patient to discriminate. Drop to 20/30 or 20/40.
- Skipping the sphere compensation: changing cylinder by 0.50 without adjusting sphere leaves the circle of least confusion off the retina. Acuity drops and the patient becomes confused.
- Refining power before axis: an incorrect axis will not give a stable power end point. Always axis then power.
- Holding the JCC with the handle 45 degrees off during axis check: this configuration is for power, not axis. Axis check requires the handle parallel to the cylinder axis.
- Patient confused by flip technique: re-explain that you are showing two views and they choose which is clearer, or report they look the same. Use "one or two, or same?" not open-ended questions.
- Walking the axis past the correct point: take small 2 to 3 degree steps as you converge. Big jumps make it easy to overshoot.
Patient Communication
Patients find the JCC confusing without coaching. Explain that you will show two views in quick succession and they should pick the clearer one or say they look the same. Stress that there is no right answer and equal-blur is a valid choice; in fact it is the goal. Speak in plain language: "Which is clearer, one (flip) or two (flip)? Or do they look the same?" Repeat each view if the patient is unsure, but do not let the patient stare for long because accommodation can drift.
For older patients with cataract or media opacity, JCC end points can be unstable. Accept best estimate and verify acuity with the proposed prescription before finalizing. For pediatric patients, JCC is generally not used; cycloplegic retinoscopy gives a more reliable cylinder.
Common Patient Questions
- "They both look blurry." Reassure that blurry is expected because you reduced the line size for the test. They are choosing between two blurs.
- "Can you do that again?" Yes, repeat as needed. Limit to two or three repeats per choice to avoid fatigue.
- "Why do you keep flipping that thing?" Brief explanation: you are fine-tuning the astigmatism part of their prescription.
Exam Tips
ABO-NCLE, IJCAHPO COA and COT, and paraoptometric (CPO/CPOA) exams test the JCC heavily because it is the standard refinement step in subjective refraction. High-yield points:
- JCC has a spherical equivalent of zero. The plus and minus cylinder components cancel.
- Axis first, then power. Always.
- For axis refinement, the JCC handle is parallel to the cylinder axis.
- For power refinement, the JCC handle is 45 degrees to the cylinder axis (so the JCC axes align with the cylinder).
- In minus-cylinder refraction, chase the red dot during axis refinement.
- For every 0.50 D change in cylinder, compensate sphere by 0.25 D in the opposite direction. This is the most-tested calculation on ABO-NCLE.
- End point is patient indifference: the two flip views look equally blurred.
- Reduce acuity to 20/30 or 20/40 before starting JCC. The standard +/-0.25 D JCC is too small to detect at 20/20.
- The white dot is plus cylinder axis, the red dot is minus cylinder axis, per ANSI Z80.
- Recheck best sphere after JCC because the spherical equivalent of the prescription may have shifted.
The Jackson cross cylinder is one of the most testable refraction skills in eye care. Its logic is simple but the orientation rules trip up new examiners. Memorize the handle-position rule for axis vs power, the chase-the-red convention, and the half-cylinder sphere compensation, and you will handle JCC questions on ABO-NCLE, IJCAHPO, and AAO board exams reliably. Pair JCC mastery with strong retinoscopy and you have the two pillars of objective and subjective refraction.