Visual acuity testing is a big part of eye care. It helps doctors and vision scientists figure out how clearly someone can see fine details. Among all the methods, the LogMAR chart is one of the most reliable. But how exactly are LogMAR values calculated?
Let’s walk through it step by step. I’ll also cover the basics of minimum angle of resolution (MAR), show you how Snellen and decimal acuity tie into the formula, and give you real-life examples.
What Is a LogMAR Chart and Why Does It Matter?
A LogMAR chart (Logarithm of the Minimum Angle of Resolution) is designed to measure visual acuity more precisely than older charts like Snellen.
- Snellen chart: Uses fractions like 20/20 or 6/6.
- LogMAR chart: Uses a logarithmic scale.
Here’s why that’s important:
- Zero (0.0 LogMAR) = normal 20/20 vision.
- Positive values (+0.3, +1.0, etc.) = worse vision.
- Negative values (-0.1, -0.3, etc.) = better-than-average vision.
Unlike Snellen, which has uneven steps between lines, LogMAR charts decrease letter size in equal, logarithmic steps. That consistency makes them perfect for research, clinical trials, and accurate low-vision assessments.
The Basics of Minimum Angle of Resolution (MAR)
To calculate LogMAR, you first need to understand MAR (Minimum Angle of Resolution).
- MAR = the smallest angle (in minutes of arc) at which the eye can tell two points apart.
- For normal vision (20/20), MAR = 1 minute of arc.
- For worse vision (like 20/40), MAR = 2 minutes of arc.
The formula is simple:
MAR = 1 / Decimal Acuity
Examples:
- Decimal acuity 1.0 (20/20) → MAR = 1
- Decimal acuity 0.5 (20/40) → MAR = 2
How LogMAR Values Are Calculated
Here’s the key formula:
LogMAR = log₁₀(MAR)
In practice, this is how it works on a LogMAR chart:
- Patient reads the chart at a standard test distance (usually 4 or 6 meters).
- Find the lowest full line the patient can read. Each line has a base LogMAR value.
- Count any extra letters correctly read on smaller lines.
- Adjust score:
LogMAR VA = Best line LogMAR – (0.02 × extra letters read)
Why 0.02? Each letter = 0.02 LogMAR. Each row = 0.1 LogMAR, and there are 5 letters per row (0.1 ÷ 5 = 0.02).
Example:
- Best full line read: 0.3 LogMAR
- Extra letters read: 4 from the next line
- Score = 0.3 – (0.02 × 4) = 0.22 LogMAR
Converting Between LogMAR, Snellen, and Decimal Acuity
Conversions are straightforward once you know the relationships.
- From Snellen to LogMAR:
- Convert Snellen to decimal: 20/40 = 0.5
- MAR = 1 ÷ 0.5 = 2
- LogMAR = log₁₀(2) = 0.3
- From Decimal to LogMAR:
- LogMAR = –log₁₀(Decimal Acuity)
Here’s a handy reference table:
| Snellen (US) | Snellen (Metric) | Decimal Acuity | LogMAR | Interpretation |
|---|---|---|---|---|
| 20/20 | 6/6 | 1.0 | 0.0 | Normal vision |
| 20/25 | 6/7.5 | 0.8 | 0.1 | Mild reduction |
| 20/40 | 6/12 | 0.5 | 0.3 | Moderate loss |
| 20/200 | 6/60 | 0.1 | 1.0 | Legal blindness |
| 20/10 | 6/3 | 2.0 | -0.3 | Excellent vision |
Why Use LogMAR Over Snellen?
LogMAR has several advantages:
- Precision – Letters and spacing are standardized.
- Consistency – Each row has equal steps.
- Flexibility – Works well for both research and clinical care.
- Averaging – Values can be averaged for statistical analysis.
That’s why professional guidelines (like from the American Academy of Ophthalmology) recommend LogMAR for accurate testing.
Practical Tips for LogMAR Testing
If you’re doing a LogMAR test:
- Make sure lighting and test distance are correct.
- For very poor vision, use counting fingers (CF) or hand movement (HM) estimates → converted into approximate LogMAR.
- Stick to validated charts (ETDRS, Bailey-Lovie).
Conclusion: Mastering LogMAR for Better Eye Care Insights
Now you know exactly how LogMAR values are calculated—from MAR basics to Snellen conversions.
LogMAR may sound technical at first, but the math is simple once you break it down:
- MAR = 1 ÷ Decimal Acuity
- LogMAR = log₁₀(MAR)
- Adjust by letters read (–0.02 per letter)
This scale gives doctors and researchers a clear, consistent way to measure sight. It’s more than just numbers—it helps guide treatment, track eye disease, and improve patient care.
This article is for informational purposes only and not a substitute for professional medical advice.
FAQs
0.5 Snellen is not a normal way to read Snellen. Snellen is shown as a fraction, like 20/40. The 20 means you are 20 feet from the chart. The 40 means a person with normal eyes can read the line at 40 feet.
0.4 is not the normal way to read Snellen. To get a Snellen fraction, you can do some math. For a 20-foot chart, 20 / 0.4 is 50. So, 0.4 is the same as 20/50 vision.
0.8 is a way to show normal vision as a decimal. It means a person can see a line from 80 feet away. A normal person can see this same line from 100 feet away. A person with 0.8 vision sees just a bit less well than normal.
You find Snellen acuity by testing your eyes. You stand 20 feet from a chart. Not only that, but you read the smallest line you can see. The last line you read is your vision.
The LogMAR chart is scored in a simple way. You read each line of letters. Each letter you get right lowers your score. A low score means better vision.
A 0.2 LogMAR score means your vision is not quite normal. A normal LogMAR score is 0. A score of 0.2 means your vision is just a bit blurry.
A 0.8 LogMAR score is equal to 20/125 in Snellen. This is a big drop from normal vision. You would need glasses to see.
To change Snellen into LogMAR, you need to use a math rule. You divide the bottom number by the top. Then you use a log function. For example, log(40 / 20) = 0.3.
You find the LogMAR value with a math rule. The rule is log(x / y). The x is the distance a normal person can see a line. The y is the distance you can see the same line.
LogMAR is better than Snellen for a few reasons. LogMAR is a log scale. Each line on the chart is the same size. LogMAR is easier to use for studies of the eye.
VAS is a user-friendly scale: VAS = 100 – 50 × LogMAR. It’s more intuitive as higher scores mean better vision.
Use the per-letter adjustment: Subtract 0.02 for each extra letter read beyond the full line.
Yes, with adaptations like larger charts or near reference tables for practical logMAR near vision assessments.
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