Objective: To determine the difference in visual acuity (VA) when using the standard Snellen chart and the novel chart (Sak charts) in the groups which have and have not been examined by the standard Snellen Chart before.
Material and Method: This research is a diagnostic study divided into two groups; groups one involves the samples which have never been examined using the standard Snellen Chart before (n = 149) and group two is done on the samples which have experienced with the Snellen Chart (n = 148). The research was conducted at Thammasat University Hospital and the area around Pathumthani province by collecting data of the general population presented with descriptive statistics and visual acuity in Decimal, LogMAR and the number of letters analysed by paired t-test and Wilcoxon Signed Ranks Test (p<0.05).
Results: In group one, there are no significant differences in VA between the standard Snellen chart and SAK Chart No. 1 with Decimal (p = 1.000 ), Log MAR (p = 1.000), no significant differences in VA between the standard Snellen Chart and SAK Chart No. 2 with Decimal (p = 1.000), Log MAR (p = 1.000), and no significant differences in VA between SAK Chart No. 1 and SAK Chart No. 2 with Decimal (p = 1.000), Log MAR (p = 1.000). In part 2, on the other hand, there are significant differences of VA between the standard Snellen Chart and SAK Chart No. 1 with Decimal (p<0.001) and Log MAR (p<0.001), significant differences in VA between the standard Snellen Chart and SAK Chart No. 2 with Decimal (p<0.001), Log MAR (p<0.001).
Conclusion: No significant differences in VA between the standard Snellen Chart and SAK Chart is found in the samples which have never been examined using the standard Snellen Chart before, but significant difference in VA is observed in the group of people who have been examined using the standard Snellen chart before.

Keywords: Visual acuity, Snellen chart, SAK chart