Contrast Card Generator

How big should it be? A plain-language guide to sizing a contrast card

Getting the size right comes down to just two numbers — how well your child sees right now, and how far away the print will be — and one line of grade-school geometry does the rest. Here's how to think about it without any math.

Mixed evidenceUpdated Jul 23, 2026For parents, educators, caregivers

Combines established science with practitioner guidance and our design opinion.

In one line

Getting the size right comes down to just two numbers — how well your child sees right now, and how far away the print will be — and one line of grade-school geometry does the rest. Here’s how to think about it without any math.

Key points

  • The eye measures angles, not centimetres. The same stripe looks “half as big” from twice as far away. So a feature’s size on paper has to grow with the distance to the surface.
  • Two inputs decide everything: (1) an acuity number — either read off a by-age table (the guided path) or typed in from an eye exam (the direct path, e.g. 20/200, logMAR, or cpd) — and (2) the viewing distance you measured.
  • We aim well below the limit, on purpose. We don’t draw at the finest detail the eye can barely catch; we draw at about a third of it so the pattern is easy and bold, sitting in the most sensitive part of a baby’s vision — not at the edge. That fraction is a single boldness knob (bolder ↔ finer).
  • Bigger is a safe error; smaller is not. Go under the minimum and the detail drops below threshold — effectively invisible. Larger than the minimum is usually fine, and often better for the youngest babies.

The numbers, in context

Minimum stripe/bar width our engine produces for a 1-month-corrected baby, so you can feel how much distance changes things:

WhereDistanceMin bar widthWhole shapeFits one A4?
Changing table~55 cm~1 cm~6 cm✅ yes
Ceiling (low)~1.5 m~2.6 cm~16 cm⚠️ borderline
Ceiling (high)~2.1 m~3.7 cm~22 cm❌ tile across sheets

Same baby, same day — the ceiling print needs bars ~3–4× wider than the changing table, and a shape too big for one sheet. As a child’s vision matures, every one of these gets finer at the same distance, so next month’s print can carry more detail.

The evidence

  • [established science / math] “Physical size ≈ distance × angle” is ordinary optics; the engine uses the exact form bar = distance × tan(½° ÷ design-frequency) (research §04.2). Nothing exotic — just trigonometry applied consistently.
  • [established science] The by-age acuity numbers (newborn ≈ 1 cycle/degree ≈ 20/400, improving quickly in the early months) come from the infant-vision literature (AAO; PMC review of acuity & contrast sensitivity), summarised in research §01.
  • [our design opinion] Designing at ~⅓ of the acuity limit (tunable ¼…½) is a deliberate safety margin so patterns land near the peak of the infant contrast curve, not at threshold (research §04.3). It’s a design choice, not a clinical rule.
  • [practitioner guidance] For a baby born early, size to corrected age, not birthday age (research §03) — using the older age makes patterns too fine to see.

Practical takeaways

Caveats & disclaimer

These sizes are conservative design targets, not a measurement of your individual child — babies vary widely. This is a non-medical enrichment tool; for premature, CVI, or low-vision care, follow your clinicians, who override anything here.