Smartphone technology is becoming widely available to all age groups and settings. Apps using Snellen acuity charts are widely available and this study evaluated the equivalence of a smartphone-based visual acuity chart with a standard 6m Snellen visual acuity (6SVA) chart. The study was conducted in two parts: Part 1 consisted of a review of available apps on iPhone and part 2 of prospective comparative study between 6SVA and iPhone visual acuity using the ‘Snellen’ application on an Apple iPhone 4. For Part 1, 11 applications were identified, with accuracy of optotype size ranging from 4.4–39.9%. Eighty-eight patients from general medical and surgical wards in a tertiary hospital took part in Part 2. The mean difference in logMAR visual acuity between the two charts was 0.02 logMAR. The largest mean difference in logMAR acuity was noted in the subgroup of patients with 6SVA worse than 6/18, who had a mean difference of two Snellen visual acuity lines between the charts. We did not identify a Snellen visual acuity app at the time of study, which could predict a patient’s standard Snellen visual acuity within one line. There was considerable variability in the optotype accuracy of apps and care must be taken when using such tools. The authors’ mention that none are currently suitable for clinical use and additional validation is required for assessing patients with severe vision impairment.