Basic vision checks are now accessible from a smartphone. Using a Snellen chart on a phone allows individuals to perform an initial self-assessment of their sight. This article guides you through the process, explaining what these tests measure, how to interpret results, and when professional eye care is necessary.
What a Snellen Chart Measures
A Snellen chart primarily measures visual acuity, the sharpness of vision at a specific distance. It assesses how well you distinguish fine details and shapes, testing your central vision used for tasks like reading or recognizing faces.
Results are typically expressed as a fraction, such as 20/20 or 20/40. The first number represents your distance from the chart (conventionally 20 feet in the U.S.). The second number indicates the distance at which a person with “normal” vision could read the same line. For example, 20/20 vision means you see at 20 feet what a person with standard vision sees at 20 feet.
A person with 20/40 vision needs to be 20 feet away to read letters a 20/20 person could read from 40 feet, indicating reduced sharpness. Conversely, 20/15 vision means you see at 20 feet what a 20/20 person would need to be 15 feet away to see, suggesting better than average visual acuity.
Using a Snellen Chart on Your Phone
To begin, select a reputable vision testing application. These apps provide a digital Snellen chart, often with adjustable settings for screen size and testing distance. Ensure your phone screen is clean and brightness is set to a comfortable, consistent level.
Find a well-lit area with minimal glare on your phone screen. Maintain a consistent testing distance from your device, which is usually specified by the app, often ranging from 10 to 20 feet. Accurately measure this distance using a measuring tape for reliable results. Positioning your phone at eye level, perhaps by propping it up or having someone hold it, helps maintain consistency during the test.
Begin the test by covering one eye completely without applying pressure, perhaps using your palm or an eye patch. With the uncovered eye, read the smallest line of letters you can clearly distinguish on the chart, starting from the top. Record the line number or fraction associated with the smallest line you successfully read. Repeat the entire process for your other eye, ensuring the same testing conditions are maintained.
Interpreting Your Vision Test Results
After completing the test for both eyes, you will have a fractional number for each eye (e.g., 20/20, 20/30, or 20/60). A 20/20 result indicates standard visual acuity at 20 feet, meaning you see details as clearly as a person with average vision. This is often regarded as clear, uncorrected vision.
If your result is 20/30, it means that at 20 feet, you see what a person with standard vision sees from 30 feet away, suggesting a slight reduction in visual clarity. A score like 20/60 implies a more significant decrease in sharpness, indicating you need to be 20 feet away to discern details a 20/20 person could see from 60 feet.
These results provide a general indication of your distance vision acuity. They serve as a screening tool rather than a definitive diagnosis of your overall eye health. While a 20/20 result is desirable, any significant deviation suggests your vision may not be as sharp as it could be.
Limitations and When to Seek Professional Advice
While using a Snellen chart on your phone offers a convenient way to check visual acuity, it has limitations. These tests primarily measure distance vision and do not assess other important aspects of eye health. For instance, they cannot detect underlying eye diseases like glaucoma, cataracts, or macular degeneration, which often progress without noticeable early symptoms.
These phone-based assessments do not evaluate specific refractive errors like astigmatism (blurred vision due to an irregularly shaped cornea or lens). They also do not test for color blindness, peripheral vision issues, or eye coordination. Accuracy can be influenced by screen resolution, ambient lighting, and precise testing distance, leading to misleading results.
Consult an eye care professional if you experience sudden vision changes (e.g., new blurriness, double vision, or flashes of light). Persistent eye pain, redness, or discharge also warrant immediate attention. If phone test results consistently show vision worse than 20/20, or a noticeable difference between your eyes, schedule a comprehensive eye examination. A professional eye exam includes tests that provide a complete picture of your eye health beyond simple visual acuity.