Humphriss Test & Binocular Balancing: Step-by-Step Method

Balance after both eyes have been refracted monocularly and before you record the final binocular acuity. Both eyes receive the same accommodative effort, so a monocular end point can carry a small error that only shows with both eyes open. Balancing makes sure neither eye is left over-minused and that the binocular result keeps the most plus for the best acuity. Methods that compare one eye's clarity with the other's assume similar best-corrected acuity in each eye; when one eye clearly sees less well, do not chase equal clarity. Humphriss never occludes an eye, which is why it suits patients whose vision changes under occlusion, such as those with latent nystagmus.

With the monocular result in the trial frame and both eyes open, place a fogging lens of +0.75 D over the left eye (some texts use +1.00 D). It should blur that eye's central vision enough to be ignored without breaking fusion; this is the 'psychological septum'. With the patient reading the chart, show the right eye +0.25 D and then −0.25 D in quick succession and ask which is clearer. Aim for the most plus that still gives the best vision: add the plus when the patient prefers it or sees no difference, and accept extra minus only when it is clearly better. As plus is accepted, accommodation relaxes in both eyes and the fogged eye can start to clear, so check that the fog still holds and increase it if it does not. The cylinder can be rechecked with the cross-cylinder at the same stage. Move the fog to the right eye and refine the left eye the same way, then remove the fog and record the binocular acuity.

In a blur-back balance with alternate occlusion, fog both eyes with about +1.00 D, cover each eye in turn, and add +0.25 D to whichever eye is clearer until the two look equally blurred; then reduce the plus binocularly in 0.25 D steps to the best acuity. Prism dissociation works the same way but shows each eye its own copy of the target: fog both eyes, place vertical prisms (for example 3Δ base up before one eye and 3Δ base down before the other) so the two lines appear one above the other, and add plus to the eye whose line is clearer until they match. Then remove the prisms and reduce the fog binocularly. The duochrome uses colour rather than blur. It depends on the eye's longitudinal chromatic aberration: green light focuses slightly in front of red, so when the black letters on the green background are clearer the eye has too much minus and needs plus, and when those on the red are clearer it needs minus. On its own that is a sphere check, not a balance: with both eyes viewing one undissociated duochrome, one eye can drive the response. To balance with it, dissociate the eyes, with prisms or polarising filters, so that each sees its own target, and bring both to the same red-green end point.

First, a fog of the wrong strength: too weak and the fogged eye keeps contributing to central vision, too strong and fusion breaks, which loses the point of Humphriss. Next, accepting minus because the letters look smaller and darker rather than because acuity improves, which can leave the patient over-minused and accommodating. Others are presenting the two lenses too slowly for a real immediate contrast, and forcing a clarity balance when it fails because the best acuities differ or the responses are unreliable. Where the acuities are comparable and exact equality falls between two lens steps, a commonly taught convention is to leave the dominant eye slightly clearer. Finally, do not skip the binocular acuity at the end, or record it loosely.

The methods you are expected to know are Humphriss, duochrome and blur-back, along with recording the final acuity in LogMAR. Each approaches the same problem differently, and knowing which to reach for, and when one of them will not work, is more useful than being able to perform all three mechanically. Be clear about what accommodation does during each technique, since accommodation is the confounder every one of these methods is built to defeat. LogMAR scoring is worth knowing properly too, including how partial lines are credited.

Practise all three techniques under supervision on patients who respond differently, because the value of understanding the principle only shows up when the standard response does not appear. In parallel, drill the underlying optics with exam-style questions so that the chromatic aberration behind duochrome and the fogging logic behind Humphriss are things you understand rather than things you recite. Use the referenced explanations to check the mechanism, not the endpoint.