Visual Field Testing: What Perimetry Measures and Why the Grid Spacing Matters
A map, not a measurement
Most eye tests produce a number. Perimetry produces a map. The machine presents a light at hundreds of positions across the visual field, one at a time, and records for each position the dimmest stimulus the patient could see. The output is a grid of sensitivity values, and every clinical decision that follows is an interpretation of that grid rather than a reading of a single figure.
That distinction matters because two people can have identical visual acuity and completely different visual fields. Acuity describes the centre; the field describes everything else, which is most of what a person uses to walk, drive and notice a cyclist approaching from the side.
The island of vision
The standard model is an island: sensitivity is highest at the fovea, falls steeply in the first few degrees, then more gradually toward the periphery. A perimetry test samples that island at fixed intervals and reports the height of the surface at each sample point.
Sensitivity is reported in decibels, and the decibel here is a logarithmic attenuation unit: each decibel corresponds to one tenth of a log unit of stimulus brightness. Because the scale is logarithmic, a drop of 10 dB means the patient needed a stimulus ten times brighter to see it, not a stimulus 10 percent brighter. This is why small-looking numbers on a field printout can represent large losses.
Why the grid spacing is the whole argument
- 24-2 pattern. Samples the central 24 degrees with points spaced 6 degrees apart. It covers enough area to catch peripheral field loss, including the arcuate patterns that follow nerve fibre bundles.
- 10-2 pattern. Samples the central 10 degrees with points spaced 2 degrees apart. It covers a much smaller area at three times the density, which is what a macular problem demands.
The arithmetic behind that difference is simple and unforgiving. A pattern that samples every 6 degrees cannot resolve a defect smaller than roughly 6 degrees across, because a point in the middle of the defect may simply not be tested. A 3-degree scotoma can sit between two tested points and produce a clean-looking field. The 10-2 pattern exists because the fovea and the immediate parafovea are exactly where that resolution is needed and where the consequences of missing a small defect are largest.
Where the 6-degree grid shows its limits
Consider a person with a small central or paracentral defect and acuity that is still reasonable, because the fovea retains a few functioning points. On a 24-2 the defect may appear as nothing at all, or as one isolated low point that looks like noise. On a 10-2 the same eye shows a coherent cluster of depressed points with a shape that matches a structural lesion. Same eye, same day, different conclusion.
The reverse failure also happens: a 10-2 pattern is blind to the periphery, so it will miss a peripheral defect entirely. The choice of pattern is a choice about which part of the island you are willing to leave unexamined.
What the test actually asks of you
The mechanics explain most bad fields. One eye is covered, the other looks steadily at a central fixation target, and a stimulus appears somewhere in the field at varying brightness. The patient presses a button when it is seen. The test is therefore a measurement of detection under divided attention: look here, respond there, continuously, for several minutes per eye.
Correction matters too. The patient wears their near correction during the test, because an uncorrected refractive error blurs the stimulus and lowers measured sensitivity across the whole field in a way that looks like generalised depression. A trial frame adds its own artefacts at the rim, which is one reason field results are read alongside the refraction rather than on their own.
Reliability is part of the measurement
Perimetry is a subjective test dressed as an objective one. The machine records fixation losses, false positives and false negatives, and those indices decide whether the map is worth reading at all. A test with poor fixation is not a slightly noisy test; it is a map of where the eye happened to be pointing, and interpreting it as a field defect is a classic error.
This is also why a single field is rarely decisive. Short-term fluctuation between two tests of the same eye is normal, and the standard practice of repeating an abnormal field before acting on it exists because the test's own variability is comparable to the defects being sought.
Three misconceptions
A normal 24-2 means the visual field is normal. It means the sampled points within 24 degrees were normal. The far periphery and, more importantly, the gaps between sample points are not covered.
The machine is objective, so the result is fact. The stimulus delivery is objective; the response is not. Reliability indices are part of the result, not a footnote.
Field loss always means disease. Refractive error, a drooping upper lid, a poorly fitting trial frame and an unmasked lens edge all produce field artefacts. That is why fields are interpreted alongside acuity, pupils and the view of the fundus rather than in isolation.
What to ask when you are given one
Ask which pattern was run and why, whether the reliability indices were acceptable, and whether a repeat is planned. If a defect has been found, ask whether a denser central pattern was added. Those three questions distinguish a field that was interpreted from a field that was merely generated, and the difference is measurable in what gets missed.