The standard Snellen chart distance is 20 feet in the US and 6 meters in metric practice. That is the distance at which the letters on the "normal" line are designed to subtend five minutes of arc. Few exam rooms are actually 20 feet long, though. Lanes reach that distance with a mirror, use a chart drawn for a shorter distance, or run a digital chart that rescales its letters to the distance you enter. This guide covers the geometry behind each option: what the standards say, how far a mirror lane really is, how to set up a digital chart or projector for the room you have, and the checks that keep acuity readings comparable from lane to lane.
It is a room-planning guide for practice owners and lane builders, not a guide to interpreting acuity. If you are choosing the chart itself, our buyer's guide to digital LCD acuity chart systems compares the models.
Shopping for digital acuity charts? US Ophthalmic is a direct provider of digital acuity charts to eye care practices across the US. Tell us how your lane is set up and we will come back with the configurations that fit, plus current availability and lead times.
Why the chart distance is fixed by geometry
A Snellen chart is drawn for one distance. Snellen defined "standard vision" as recognizing one of his letters when it subtends 5 minutes of arc. The strokes of each letter, and the gaps between them, subtend 1 minute of arc. At 6 meters, that puts the 6/6 (20/20) letters at 8.73 mm tall and the top 6/60 "E" at 87.3 mm.
The fraction records that distance. In 6/x or 20/x notation, the top number is the distance between the patient and the chart, and the bottom number is the distance at which a person with 6/6 acuity would just read the same letter. Move the patient and the angles change, so the line labels stop being true unless the letters are rescaled.
Because acuity is defined by angle, it can be measured at any distance as long as the letter sizes are scaled to match. That is why charts exist for 3 m, 2.5 m and 2 m, and why a digital chart can work in a short room. What changes with distance is how much the eye has to focus, and how much a small lean forward throws the reading off.
What the standards say about test distance
There is no single required distance. The International Council of Ophthalmology's Visual Acuity Measurement Standard says: "At this time there is no single uniformly accepted testing distance." It suggests 4 meters as the standard, prefers metric specification, and accepts clinical testing at 5 and 6 meters. It also states plainly: "Mirror systems are accepted."
- 20 feet / 6 meters is the traditional Snellen distance and the one most US charts are labeled for. The ICO standard notes that "few examining rooms are built for a 6-meter or 20-foot distance today."
- 4 meters is the ICO's suggested standard. The NEI ETDRS protocol also cut the viewing distance to 4 meters "to have a more manageable chart size," according to chart maker Precision Vision. The NAS-NRC Committee on Vision guidelines, widely followed in the USA and Europe, also cover distance of use, lighting, luminance and calibration for computerized charts.
- 10 feet / 3 meters is common for children, since it is easier to hold their attention, and printed charts are sold in double-sided 20 ft / 10 ft versions. Charts for 2.5 m and 2 m exist for shorter lanes.
The ICO also asks that the testing distance used be specified. Whatever distance a lane runs at, write it into the lane documentation and the chart settings, so a reading taken in Lane 1 means the same as one taken in Lane 3.
Shorter lanes and accommodation: the numbers
A dioptre is a reciprocal meter, so the focusing demand of a target is 1 divided by its distance in meters. None of the usual lane lengths is optical infinity. The ICO standard says so directly: "Neither a 4, nor a 5 or 6 meter testing distance represents true optical infinity." The table shows what each distance asks of the eye and how big a 20/20 letter has to be at that distance (8.73 mm at 6 m, scaled in proportion).
| Test distance | Vergence at the eye (1 ÷ distance) | 20/20 (6/6) letter height | Where it is used |
|---|---|---|---|
| 6 m (≈20 ft) | 0.17 D | 8.73 mm | Traditional Snellen lane; mirror lanes built to 20 ft |
| 5 m | 0.20 D | 7.28 mm | Accepted by the ICO standard |
| 4 m | 0.25 D | 5.82 mm | ICO suggested standard; ETDRS protocols |
| 3 m (≈10 ft) | 0.33 D | 4.37 mm | Half-size charts; pediatric testing |
| 2 m | 0.50 D | 2.91 mm | Short-lane charts |
| 1.5 m | 0.67 D | 2.18 mm | Short end of some digital charts |
| 1 m | 1.00 D | 1.46 mm | Shortest setting on some digital charts |
Two things follow for lane design.
The difference between 4, 5 and 6 m is small. The ICO standard notes that the steps for 5 m (0.20 D) and 6 m (0.167 D) are not even available in trial lens sets. For 4 m testing it describes adding a 0.25 D negative correction to the refractive result to make the eye optically conjugate with infinity. Research protocols do the same thing: a Bausch & Lomb trial protocol places a +0.25 D lens in front of the eye when testing uncorrected distance acuity at 4 meters, "to correct for the difference between the testing distance and optical infinity." How your doctors handle this is a clinical policy, not a lane-design decision. The point for the room is that the lane distance has to be known and fixed for any such rule to work.
Below about 3 m, the demand climbs fast and small movements matter more. Precision Vision notes that the longer viewing distances "relax accommodation" and that "small forward movements of the patient have a negligible influence." At 1 m, a typical chart "requires 1 D over the distance correction." In a short room, a patient who sits forward 15 cm changes the true distance by a larger share of the total than in a 6 m lane. Plan short lanes with a fixed chair position for that reason.
Mirror lanes: how far the patient is really looking
A mirror folds the lane. A plane mirror forms an image that appears behind the mirror, as far behind it as the object is in front, so the viewing distance is the eye-to-mirror distance plus the chart-to-mirror distance. A patient seated 3 m from the mirror, looking at the reflection of a chart mounted 3 m in front of the mirror, views it at 6 m.
Mirror images are laterally inverted, so the chart has to be reversed. Printed mirror charts are made with reversed letters for this purpose, and digital charts offer a mirror setting (more on that below). The chart faces the mirror, so it sits on the wall at the patient's end of the room, usually above and behind the chair.
| Eye to mirror | Chart to mirror | Viewing distance | Use |
|---|---|---|---|
| 3.05 m (10 ft) | 3.05 m (10 ft) | 6.1 m (20 ft) | Classic 20 ft chart in a 10 ft lane |
| 3.0 m | 3.0 m | 6.0 m | Metric 6 m chart |
| 2.0 m | 2.0 m | 4.0 m | 4 m chart in a small room |
| 2.5 m | 3.0 m | 5.5 m | Digital chart: enter 5.5 m, not 6 m |
The last row is the common mistake. When the chart cannot hang directly over the patient's head, the two legs are not equal and the lane is shorter or longer than its nominal distance. Measure both legs with a tape after installation and use the sum.
Mirror lanes are accepted by the ICO standard, and research protocols use them too. One ETDRS-style protocol notes that reverse charts are available "for rooms that require 'folding' via a mirror," and that if the chart cannot sit at exactly 4.0 m, the site must "record the actual test distance used."
Digital LCD charts: calibrate to the distance you measure
A digital chart solves the room-length problem by redrawing the letters for whatever distance you give it. The Ezer EDC-BH spec sheet says its charts "automatically resize based on the working distance (1.0 – 6.0 m) entered by the user," in 0.1 m steps. The Ezer EDC-2600 lists a distance range of 1.5 to 7.5 m in 0.25 m steps, plus a mirror mode. The Luxvision LDC-2600 page says its chart size "automatically calibrates to the working distance, eliminating the need for mirror systems."
The key phrase is "entered by the user." The chart cannot measure the room. If the distance in the settings is wrong, every letter is the wrong size and every line label is off by the same ratio. Set it up like this:
- Fix the chair position first. Mark it on the floor. One research protocol requires the testing distance to be "marked clearly and permanently on the floor."
- Measure from the patient's eyes to the screen. Protocols measure "between the participant's eyes and the visual acuity chart," with the patient's back firmly against the chair. Measure with a seated adult in the chair, not from the chair's front edge.
- For a mirror lane, measure both legs and enter the sum. Then turn on the chart's mirror mode so the letters read correctly in the reflection.
- Round to the chart's step, then label the lane. The EDC-BH accepts 0.1 m steps and the EDC-2600 0.25 m steps. Record the entered distance on the lane checklist so the next tech does not "correct" it.
- Re-check after any move. A new chair, a re-hung mirror or a moved stand changes the distance. One multicentre study using computerized charts notes the 4 m viewing distance "was always checked before starting the examination." That is a good habit to copy.
Very short lanes and screen resolution
At short distances the letters get small, and a screen draws them with a fixed number of pixels. The Luxvision LDC-2600 lists a pixel pitch of 0.248 mm. A 20/20 letter at 3 m is 4.37 mm tall, about 18 pixels on that panel, with strokes about 3.5 pixels wide. At 1.5 m, the same letter is about 9 pixels tall with strokes under 2 pixels, and lines smaller than 20/20 shrink further. That is our arithmetic from the published pitch and the 5-arc-minute letter definition, not a manufacturer rating. If your lane is near the bottom of a chart's range and you test the smallest lines, ask the manufacturer what the shortest supported distance is for those lines, and view the 20/15 and 20/10 lines at the lane distance before you sign off on the install.
Chart projectors: throw distance and the screen
A projector puts the chart on a screen at the end of the lane, and its focus range sets the lane lengths it supports. The Luxvision LCP-7800 lists a projection distance of 2.5 to 8 m, 41 charts, 34 masks, and red/green and polarization filters. Other projectors list comparable ranges; Essilor's CP 550 leaflet, for example, gives 2 to 7 m.
Three setup points are specific to projectors:
- Set it for the lane distance. Projectors are specified for a range of projection distances, so set yours up for the measured screen distance following its manual. Handle the projection distance like a digital chart's distance setting: measure it, set it, record it.
- Match the screen to the polarized tests. Projectors that run polarized tests are sold with polarized metal screens as accessories. Topcon lists a "Polarized Metal Screen" for its ACP-8R, for example. If you plan to use the polarization filters, confirm with the projector maker which screen to use before ordering.
- Control ambient light. The ICO standard notes that for projected charts in particular, contrast loss from ambient light "should be avoided or at least minimized." Keep stray light off the screen.
Luminance, contrast and glare
Acuity readings depend on how bright the chart is, so the lane plan should include lighting, not just distance.
- Chart luminance: the ICO standard reports US practice at 85 cd/m² and says chart luminance should not be less than 80 cd/m². The UK standard BS 4274-1 calls for at least 120 cd/m². Clinical-trial protocols commonly specify 85 cd/m² with an accepted range of 80–110 cd/m².
- Display headroom: digital charts publish a maximum brightness; the LDC-2600 lists 250 cd/m², and Reichert's ClearChart 4 lists 220 cd/m² maximum and an 85 cd/m² photopic setting. Headroom above 85 cd/m² is useful; running a lane at full brightness is not the same as running it at a defined level.
- Contrast and glare: the ICO standard says black letters should not exceed 15% of the luminance of the white background, and that tests should be installed so glare and reflections are minimized. Keep windows and overhead lights out of the mirror's line of sight and off the screen.
- Adaptation: give patients time to adapt to the room lighting before testing, as the ICO standard recommends.
We found no standard that sets an adult chart mounting height, so follow your chart maker's installation guide. For a mirror lane, hang the mirror so the seated patient sees the whole chart without tilting their head.
Choosing a setup for the room you have
| Room situation | Practical setup | What to watch |
|---|---|---|
| Straight lane of 6 m (20 ft) or more | Wall chart, digital chart or projector at full distance | Lighting and glare; record the true eye-to-chart distance |
| Room around 3–3.5 m long | Mirror lane with a reversed chart, or a digital chart in mirror mode | Measure both legs; enter or label the sum, not the nominal 20 ft |
| Room around 4 m long | Direct 4 m lane (the ICO's suggested standard) | Record 4 m as the lane distance; keep it consistent across lanes |
| Short room under 3 m | Digital chart set to the measured distance, fixed chair position | Higher focusing demand; small-line resolution near the bottom of the range |
| Several lanes of different lengths | Digital charts set per lane, with the distance on each lane's checklist | Same chart family and settings, so readings compare across lanes |
If you are planning the room from scratch, set the lane distance before you plan the furniture. Our new optometry practice equipment checklist covers how the acuity system fits with the chair, stand and refractor in the first lane, and why it pays to rough in power, data and wall distance for a second lane during construction.
Lane distance installation checklist
- Lane distance chosen and written down (6 m / 20 ft, 4 m, or the measured digital-chart distance).
- Chair position marked on the floor; measured from the seated patient's eyes.
- Mirror lanes: both legs measured with a tape; sum recorded; chart reversed or mirror mode on.
- Digital chart: distance entered to the chart's step size; smallest lines viewed at the lane distance.
- Projector: focused and sized at the measured screen distance; polarized screen confirmed if polarized tests will be used.
- Chart luminance set to a defined level (85 cd/m² is US practice; not less than 80 cd/m² per ICO).
- No windows or lights reflected in the mirror or on the screen.
- Distance re-checked after any furniture, mirror or chart move.
US Ophthalmic manufactures the Ezer line, including the EDC-BH and EDC-2600 digital charts, and backs what we sell with an in-house tech and spare-parts department and US warranty and service. If you are fitting out a lane and want help matching a chart or projector to the room you have, request a quote and include your room dimensions.
Ready to spec your digital acuity charts?
Send us your requirements and our equipment specialists will come back with a configuration built around how your practice actually works — including availability and lead time. US Ophthalmic is a direct provider to eye care practices nationwide.
Frequently asked questions
What is the standard Snellen chart distance?
20 feet in the US and 6 meters in metric practice. At that distance the letters on the 20/20 (6/6) line subtend 5 minutes of arc, which makes them 8.73 mm tall. The ICO's Visual Acuity Measurement Standard suggests 4 meters as the standard and accepts 5 and 6 meters, so the right answer for a lane is the distance the chart is drawn or calibrated for.
Can I use an eye chart at 10 feet?
Yes, if the chart is drawn for 10 feet (3 meters) or is a digital chart set to that distance. Printed Snellen charts are sold double-sided for 20 ft and 10 ft testing. A 20 ft chart read directly at 10 ft gives wrong line values; at 10 ft the same chart only works through a mirror, which doubles the path to 20 ft.
How does a mirror exam lane work?
The patient views a reversed chart in a mirror. The viewing distance is the distance from the patient's eyes to the mirror plus the distance from the chart to the mirror, so a patient 10 ft from the mirror, looking at a chart 10 ft in front of it, is testing at 20 ft. Measure both legs after installation; if they are unequal, use the sum.
What distance should I enter in a digital acuity chart?
The measured distance from the seated patient's eyes to the screen, or for a mirror lane, the full folded path. Round it to the chart's step size (0.1 m on the EDC-BH, 0.25 m on the EDC-2600), turn on mirror mode if there is a mirror, and record the value on the lane checklist.
Does a shorter lane change the refraction?
Every practical lane is closer than optical infinity: 6 m is about 0.17 D, 4 m is 0.25 D and 3 m is 0.33 D. The ICO standard describes a 0.25 D adjustment for 4 m testing, and trial protocols apply it the same way. How a practice handles this is a clinical decision; the lane's job is to have a known, fixed distance so that any policy is applied consistently.
Should I buy a new or refurbished acuity chart?
US Ophthalmic sells new equipment, not refurbished or used units. If budget is the concern, look at our open-box units: demo and display units, lightly handled returns and overstock, inspected and function-checked, with the warranty term varying by unit.
