4.2 Manual refraction assistance (JCC, duochrome, endpoint)

The phoropter (refractor)

The phoropter houses spheres, cylinders, a Jackson Cross Cylinder, prisms, and accessory lenses (pinhole, Maddox rod, red-green filters, and a plus/minus 0.50 split for balancing) behind two apertures. You dial sphere in 0.25 D steps, cylinder in 0.25 D steps, and axis in degrees. The patient views a chart at 6 m (20 ft) or through a mirrored lane. Center the patient, set the PD, and level the instrument so both eyes look through the optical centers.

Refining the sphere: maximum plus to best VA

The cardinal rule of subjective refraction is maximum plus, least minus, that gives best visual acuity. Over-minusing lets the patient accommodate to clear the letters, causing eyestrain and an unstable Rx. Procedure: fog the eye with excess plus so the chart blurs, then reduce plus (add minus) 0.25 D at a time, stopping the instant acuity stops improving. If the next -0.25 does not sharpen a smaller line, leave it out.

Fogging and avoiding over-minus

Start each eye fogged about +1.00 D beyond the retinoscopy sphere; the chart should blur to roughly 20/40 to 20/60. Reduce plus 0.25 D at a time, having the patient read down the chart. Stop at the least minus that reads the smallest line, because chasing a blacker or smaller image with extra minus only recruits accommodation. If a patient keeps asking for more minus, re-fog and repeat; genuine improvement, not a darker image, defines the stopping point.

Jackson Cross Cylinder (JCC)

The JCC is a lens with equal-magnitude plus and minus cylinders at right angles (commonly plus/minus 0.25 D), mounted so it can flip about a handle. Its markings: red dots mark the minus-cylinder axis, white dots the plus-cylinder axis, and the handle sits 45 degrees from both. Use it to refine an existing cylinder, first axis, then power.

Refining axis first

Place the JCC handle aligned with the correcting-cylinder axis so the cross-cylinder axes straddle it at 45 degrees. Flip and ask "which is clearer, one or two?" Then rotate the cylinder axis toward the minus-cylinder (red) dots of the preferred flip position, in 5-10 degree steps. Repeat until the two flip choices look equal; that equality is the correct axis.

Refining power second

Now align the JCC axes with the correcting-cylinder axis (red dot on axis, then flip). If the flip that adds minus cylinder is preferred, increase the cylinder power; if the other flip is preferred, decrease it, in 0.25 D steps. Key coupling rule: for every 0.50 D you change the cylinder, adjust the sphere 0.25 D in the opposite direction to hold the spherical equivalent and keep the circle of least confusion on the retina. Endpoint: both flips equal.

Worked example: refining -2.00 -1.00 x 180, you add -0.50 cylinder to reach -1.50; to preserve SE you add +0.25 sphere, giving -1.75 -1.50 x 180 (SE unchanged at -2.50).

Duochrome (red-green) test

The duochrome exploits chromatic aberration: red light focuses slightly behind green in the eye. Letters are shown on split red and green backgrounds.

• Letters clearer on RED: the eye is undercorrected myope / overcorrected hyperope (focus in front), so add minus.
• Letters clearer on GREEN: the eye is overcorrected myope / undercorrected hyperope, so add plus.
• Endpoint: letters equally clear on both.

Mnemonic: RAM-GAP (Red Add Minus, Green Add Plus). Because it depends on wavelength, not color perception, it works for color-deficient patients. Start slightly fogged (green favored) and reduce plus until balanced.

Binocular balance and the endpoint

After each eye is refined monocularly, balance ensures equal accommodative relaxation between the two eyes. A common method: fog both eyes about +0.75 to +1.00 D, then dissociate with prism or alternating occlusion and equalize the blur so both eyes are equally fogged, adding plus to the clearer eye until they match. Balance adjusts sphere balance, not necessarily best acuity.

The refraction endpoint is the maximum plus, least minus, that yields best corrected acuity with the two eyes balanced. Confirm with duochrome, then record sphere, cylinder, axis, add, and the acuity achieved.

Subjective refraction sequence (summary)

Accessory checks

The pinhole distinguishes refractive blur from pathology: if acuity improves through a pinhole, uncorrected refractive error remains; if it does not, suspect a media or retinal problem. When no cylinder is yet in place, you can find astigmatism with a clock dial (astigmatic fan): fog the eye, ask which spoke line looks darkest and sharpest, then add minus cylinder at the axis 90 degrees away from that clearest spoke. Always recheck the sphere after finalizing cylinder, because adding cylinder shifts the spherical equivalent and can leave the eye slightly over-minused.

Vertex distance

For powers beyond about plus or minus 4.00 D, record the vertex distance (typically 12-14 mm from cornea to back lens surface). Because a lens moved closer to the eye acts more plus and one moved farther acts more minus, a strong phoropter Rx must be vertex-compensated when it is converted to spectacles or contact lenses. A -8.00 D phoropter finding, for instance, becomes weaker in minus as a contact lens sitting on the cornea.