EyeChart App for iOS
'A Snellen Eye Chart is an eye chart used by eye care professionals and others to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen who developed the chart in 1862. While the classic Snellen chart is typically placed at 20' or 6m, pocket-sized versions such as this one can provide useful screens for rough visual acuity.'
This is a free app