The best measurable acuity depends on the viewing distance and the pixel density of the display used to perform the assessment. This widget provides the best measurable acuity when using an iOS device as a function of the viewing distance. While all iOS devices are appropriate for measuring far visual acuity, only the iOS devices with the so--called "Retina Display" can be used to measure near visual acuity in the normal population (viewing distance to assess near visual acuity is typically about 40 cm).