Sir,

We thank Rodríguez-Vallejo1 for the comments on our paper.2 The issues raised about the inadequacies of the Snellen Visual Acuity chart as a measure of vision are well founded, and we agree there are far better tools as highlighted in our paper.2 However, the scope of our paper was to compare the smartphone visual acuity applications with the visual acuity measures most commonly used on a day-to-day basis in clinical practice. It has been our experience that the Snellen visual acuity chart is used far more frequently than any other standardised chart in clinical practice by physicians (including non-ophthalmologists such as general practitioners and emergency departments).

Our formula for calculation of optotype size is also based on the arcminutes subtended by each letter, where we have calculated the ideal optotype size for a standard letter and combined this with modifiers based on VA measure and distance from the chart. When tested, both formulas yield similar results.

With regard to Rodríguez-Vallejo's finding that the ‘Snellen’ app is more inaccurate on an iPhone 6, as mentioned in the letter, it is likely due to the non-responsive design of the application, leading to different results from our study, where we specifically used an iPhone 4 for all data collection. Tablets offer an exciting opportunity for visual acuity measurement, with many well-developed applications for visual acuity testing.3 For the purposes of this study, we chose to focus on smartphones, as these are carried ubiquitously by medical practitioners, and, anectodally, in our practice we noted that physicians used smartphone apps to check visual acuity more frequently than tablets.

Tests of vision using both smartphones and tablets is a rapidly developing area, and we look forward to reviewing the latest developments. We strongly believe there is a need for greater medical input when developing these apps, especially in the light of the recent FDA guidance.