Landolt Ring Size and Distance: Why the Gap Is 1.5 mm, and How to Make a 3 m or 2.5 m Eye Chart
The C-shaped Landolt ring used in eye tests isn't drawn at a random size. At 5 m, the 1.0 ring has a 1.5 mm gap and a 7.5 mm outer diameter: its size is fixed precisely by acuity and distance. Once you understand how, you can make your own 3 m or 2.5 m eye chart without a 5 m room, and you'll see the logic behind measuring vision on a phone screen. This article covers the rules for the ring's shape, the relationship between 1 minute of arc and visual acuity, formulas and a quick-reference table for size by distance and acuity, and how to print a home eye chart at the correct size.
What a Landolt ring is: outer diameter, stroke width and gap in a 5 : 1 : 1 ratio
The Landolt ring is an eye test symbol (optotype) devised by the French ophthalmologist Edmund Landolt at the end of the 19th century. Unlike optotypes based on letters or numbers, it can be used under the same conditions with children who can't read yet or people who don't know the local script, so it's widely used in Japanese eye clinics, schools and driving licence tests, and it's also an international standard optotype.
Its shape follows fixed rules: the stroke width and the gap are the same, and the outer diameter is 5 times that. So outer diameter : inner diameter : gap = 5 : 3 : 1. What an acuity test asks is whether you can make out the gap, so the calculation starts with the gap width, and the outer diameter is simply 5 times it.
| Part | Ratio | 5 m, 1.0 ring (theoretical) | Commercial 5 m eye chart |
|---|---|---|---|
| Gap width | 1 | 1.45 mm | 1.5 mm |
| Stroke width | 1 | 1.45 mm | 1.5 mm |
| Inner diameter | 3 | 4.36 mm | 4.5 mm |
| Outer diameter | 5 | 7.27 mm | 7.5 mm |
Acuity 1.0 means being able to resolve 1 minute of arc
Acuity is determined by the angle (visual angle) that the smallest gap you can make out subtends at your eye. One sixtieth of a degree is "1 minute of arc (1′)", and being able to resolve a gap of 1 minute is acuity 1.0. Only managing a 2-minute gap is 0.5, and 10 minutes is 0.1. Decimal acuity is the reciprocal of the smallest visual angle you can resolve, in minutes.
The width that subtends 1 minute at 5 m is 5,000 mm × tan(1′) ≈ 5,000 × 0.000291 ≈ 1.45 mm. That's why people say "at 5 m, 1.0 is a 1.5 mm gap": many Japanese 5 m eye charts round it to 1.5 mm with a 7.5 mm outer diameter for convenience. That's about 3% larger than the theoretical value, so the same person will find a commercial chart very slightly easier to read.
Landolt ring sizes by distance
Size is proportional to distance and inversely proportional to acuity. Halve the distance from 5 m to 2.5 m and the same 1.0 ring only needs to be half the size. For common distances, here are the 1.0 gap and outer diameter and the outer diameter of the largest ring, 0.1 (calculated values, to 2 decimal places).
| Distance | 1.0 gap | 1.0 outer diameter | 0.1 outer diameter | 2.0 gap |
|---|---|---|---|---|
| 5 m | 1.45 mm | 7.27 mm | 72.7 mm | 0.73 mm |
| 3 m | 0.87 mm | 4.36 mm | 43.6 mm | 0.44 mm |
| 2.5 m | 0.73 mm | 3.64 mm | 36.4 mm | 0.36 mm |
| 2 m | 0.58 mm | 2.91 mm | 29.1 mm | 0.29 mm |
| 1.5 m | 0.44 mm | 2.18 mm | 21.8 mm | 0.22 mm |
| 1 m | 0.29 mm | 1.45 mm | 14.5 mm | 0.15 mm |
| 30 cm (near) | 0.087 mm | 0.44 mm | 4.4 mm | 0.044 mm |
For other acuity steps, divide the 1.0 value by the acuity. At 3 m, for example, the gap for 0.7 is 0.87 ÷ 0.7 ≈ 1.25 mm, and for 0.3 it's 0.87 ÷ 0.3 ≈ 2.91 mm. You can also see a size table for each distance in the "Conversion table & standards" tab of the Visual Acuity Test (Landolt Ring) tool.
The limits of drawing on a screen: a gap finer than a pixel can't be drawn
Drawing rings on a phone or computer screen comes with a limit that paper doesn't have. A screen is made of tiny dots (pixels), so once the gap is narrower than 1–2 pixels it can't be drawn in the correct shape. Recent phones have a fine pixel pitch of about 0.05–0.07 mm, whereas many laptops and external monitors have coarser screens at about 0.15–0.3 mm.
- To measure finer steps: move further away so the rings get larger and can be drawn even on a coarse screen (2–3 m for a computer)
- If the large steps don't fit on screen: move closer or use a device with a bigger screen (at 5 m, the 7.3 cm outer diameter of the 0.1 ring won't fit on a phone)
- Calibrate 1 mm on screen: the same "100 px" is a different physical length on different devices, so calibrate the real size with a card or coin before drawing
The Visual Acuity Test (Landolt Ring) tool automatically leaves out steps whose gap would be narrower than 2 of the device's pixels and steps that wouldn't fit within 90% of the screen's short side. Tips for measuring on a screen are collected in how to do an eye test at home: how far away should your phone be?.
Printing a 3 m or 2.5 m eye chart at the correct size
In a home without 5 m of space, printing a 3 m or 2.5 m chart is the easiest option. Paper has no pixel limit, so you can measure fine steps accurately. But if the print scale is off, the rings will be the wrong size too, so print with the following steps.
- 1
Choose a distance and create the chart
In the tool's "Print eye chart", choose a distance such as 3 m, 2.5 m or 2 m to get an A4 portrait chart with rings from 0.1 to 2.0 sized for that distance. The 5 m chart is split over 2 sheets because the large steps don't all fit.
- 2
Print at "100% (actual size)"
Turn off "Fit to paper" and "Shrink to fit page" in the print dialog. Some printers automatically shrink to around 95%, so always check the scale setting.
- 3
Measure the 10 cm line with a ruler
If the check line at the bottom of the sheet measures exactly 10 cm, the rings are the correct size too. If it's 9.7 cm, the print is 3% too small, so fix the scale and print again.
- 4
Put it up at eye level and measure the distance with a tape measure
Stick the chart on a well-lit wall at about eye height. Choose a spot where window light doesn't reflect off the paper, mark the distance on the floor and test one eye at a time.
How the Landolt ring differs from other optotypes (hiragana, E, letters)
| Optotype | Where it's commonly used | Features |
|---|---|---|
| Landolt ring | Eye clinics, schools and driving licences in Japan | You only answer the direction (4 or 8 directions), so knowledge of letters doesn't matter |
| Tumbling E | Tests for children abroad and people who can't read | You say which way the E faces. Like the Landolt ring, it's built from strokes that subtend 1 minute of arc |
| Letters (Snellen, ETDRS) | Eye clinics in the US, UK and elsewhere | You read the letters aloud. Results are written as 20/20, 6/6 or logMAR |
| Hiragana or pictures | Tests for young children in Japan | Results tend to depend on how familiar the child is with the characters or pictures |
Eye charts abroad usually express results as 20/20 (feet), 6/6 (metres) or logMAR. Conversions to Japanese decimal acuity are collected in what 20/20 vision is in Japanese acuity: a decimal acuity and logMAR conversion table.
FAQ
- Why is the Landolt ring gap 1.5 mm?
- Because the width that subtends 1 minute of arc (1/60 of a degree) at 5 m is about 1.45 mm. Being able to resolve a 1-minute gap is defined as acuity 1.0, so 5 m eye charts generally round the 1.0 gap to 1.5 mm (7.5 mm outer diameter).
- How big are the Landolt rings on a 3 m eye chart?
- The 1.0 ring has a 0.87 mm gap and a 4.36 mm outer diameter, and the 0.1 ring has a 43.6 mm outer diameter. That's exactly 0.6 times the 5 m size. For other steps, divide the 1.0 value by the acuity.
- What happens if I use a 5 m eye chart at 2.5 m?
- The rings look twice as large, so your result comes out at twice your actual acuity. Multiply the value of the line you could read by 2.5 ÷ 5 = 0.5 to get the correct acuity.
- How can I check that a printed eye chart is the right size?
- Print a line or frame of known length along with it and check its length with a ruler. The tool's eye charts include a 10 cm line, so if it measures exactly 10 cm, the scale is right. If it's off, set the scale to "100%" in the print dialog and print again.
- What is the ratio of a Landolt ring's outer diameter to its stroke width?
- Outer diameter : inner diameter : gap = 5 : 3 : 1, and the stroke width is the same as the gap. Once the gap width is set, the outer diameter is 5 times it and the inner diameter 3 times.
Tools in this article
Related articles
How to Do an Eye Test at Home: How Far Away Should Your Phone Be? 7 Tips for Accurate Results
"Distant writing has been looking blurry lately." "My licence renewal is coming up; will I pass?" Often you want to check for yourself before going to an eye doctor. Even on a phone or computer screen, if you match the size of the Landolt ring (the letter C) to the distance correctly, you can estimate your visual acuity on the same principle as a clinic's eye chart. This article covers how to do an eye test at home, how far to stand from a phone, a computer or a printed chart, why results drift and 7 tips for accurate measurement, how to read your numbers, and signs that you should see an eye doctor.
Read article →Eyesight Requirements for a Japanese Driver's License: 0.7 with Both Eyes and 0.3 per Eye for a Standard License, Other Classes, and Checking at Home Before Renewal
When the renewal notice arrives, many people start to worry: "Things look a bit blurry lately. Will I pass the eye test?" In Japan the eyesight standard depends on the license class. For a standard car license you need 0.7 or better with both eyes and 0.3 or better in each eye (decimal visual acuity, where 1.0 is normal vision). This article covers the standards for mopeds, standard, large and Class 2 licenses, the rule for people who see poorly in one eye, the depth perception test required for large and Class 2 licenses, how the test works at the license center, what to do if you fail, and how to check at home before renewal.
Read article →What Do Grades A, B, C and D Mean in a Japanese School Eye Test? The 370 Method and Whether a B Means Seeing an Eye Doctor
Your child brings home the results of the school eye test, and you wonder: "Is a B bad?" "Should we see an eye doctor?" Japanese schools test eyesight with the "370 method" (san-nana-maru), which sorts children into four grades, A to D, using Landolt rings in just three sizes: 0.3, 0.7 and 1.0. This article explains what acuity each grade stands for and how well the child sees in the classroom, what to do after a B or C, what an eye doctor checks, everyday habits that help prevent declining eyesight, and how to check your child's vision at home.
Read article →