Self-diagnosis with a Mirror for testing distant visual acuity
When you are alone you may still test your distant visual acuity using the app and a mirror.
After setting the mirror toggle 
of the Snellen/ETDRS/Numbers Chart and the auto-scoring mode, you stand in front of a mirror, 1 meter away which effectively gives a 2-meter test distance. Then you put your phone/tablet near one of your ears with the screen facing the mirror. To ensure only one eye opened, you may cover the other eye with an “eye patch”.
of the Snellen/ETDRS/Numbers Chart and the auto-scoring mode, you stand in front of a mirror, 1 meter away which effectively gives a 2-meter test distance. Then you put your phone/tablet near one of your ears with the screen facing the mirror. To ensure only one eye opened, you may cover the other eye with an “eye patch”.
You should be able to use Tumbling E Chart, Landolt C Chart and HOTV Chart in front of a mirror naturally. For example, the E on the screen has limbs pointing right, and the E reflected in the mirror has limbs pointing left, you may swipe right on the screen to mark a correct answer, and this action is mirrored as swiping left in the mirror.