From the Research
The Snellen chart does not have a specific number of letters you can get wrong on each line to pass the test, instead, visual acuity is determined by the smallest line where you can correctly identify the majority of letters, typically at least 50-60% of them. For example, if a line has 5 letters, you would generally need to identify at least 3 correctly to be considered able to see at that acuity level 1. Eye care professionals record your visual acuity based on the smallest line you can read with acceptable accuracy, expressed as a fraction like 20/20 or 6/6.
Some studies have compared the Snellen chart with other visual acuity charts, such as the ETDRS chart, and found that visual acuity scores were significantly better on ETDRS charts compared to Snellen charts, especially in patients with poor visual acuity or certain eye conditions like exudative age-related macular degeneration 2. However, the Snellen chart remains a widely used and accepted method for assessing visual acuity, and its results are generally considered reliable and valid 3, 4.
During an eye exam, the examiner may allow you to make a few mistakes on a line and still consider you to have that level of acuity, especially if you can identify some letters on the next smaller line. This flexibility exists because visual acuity exists on a continuum rather than in discrete steps, and factors like contrast, lighting, and fatigue can affect performance during testing. The most recent study on the topic, from 2020, highlights the importance of accurate visual acuity assessment, but does not provide specific guidance on the number of letters that can be missed on each line 1.
In clinical practice, the key consideration is to determine the patient's visual acuity as accurately as possible, taking into account any limitations or variability in the testing method. By using the Snellen chart and considering the patient's ability to correctly identify the majority of letters on each line, eye care professionals can provide an accurate assessment of visual acuity and make informed decisions about treatment and management.