Settings Explained

This page further explains in addition to what the in-app help has already described.


The brightness of the app is independent of the device's brightness setting. Almost all apps on a device are designed to be used at close distance less than half meter, and for the sake of eye health, many people would turn the brightness to an acceptable level. 

VAC is for testing distant visual acuity at a distance over 2 meters, and the luminance of presentation shall be uniform and not less than 120 nits (cd/m2, Candela per Square Meter), therefore commonly the brightness of the app should be brighter than the device's brightness that you get used to at close distance, or not less than half of the max brightness of the device. Please check Tech Specs of your device.

According to, the max brightness of iPhone range from 500 nits (iPhone5) to 2,000 nits (iPhone 15 Pro Max). Samsung Galaxy S22 hit 1,300 nits while S22+ and S22 Ultra hit 1,750 nits.

Keep Screen On

A device has a "Screen timeout" (or Auto-Lock on iPhone) setting upon a timespan of inactivity. When "Keep Screen On" is turned on and the app is running on the foreground, the screen is kept on.


  • The change on the setting will take effect after restarting the app: terminate the app and start again.
  • If VAC is not running on the foreground, this setting has no effect, that is, whether the screen is on depends on the "Screen timeout" setting of the device.

Voice Hint

Having this on, VAC will read the optotype that the subject should read, or the direction of the optotype that the subject should indicate.

In the Doctor and Patient mode, the doctor listens through a earphone while watching the subject or listening to the subject. This feature is especially handy when testing with Tumbling E or Landolt C when the subject is using hand gesture to indicate directions, and the doctor does not have to turn the head often between the screen and the subject.


When this setting is on, the vibration indicating an effective swipe is off.


While you may choose the conventional notation in your country, VAC always calculates the VAC while keeping the result of the non-VAC notation. VAC is more intuitive than other notations when illustrating the progress of visual acuity.

Unit of Length

When the app is firstly installed, the initial setting is Meter unless the region of the device is United State of America where the imperial foot is widely used.

Custom Distance

Custom distance could be 0.5 - 30 meters, or 1.6 - 98 ft. After setup one, the distance will appear in the Distances list. It may be fun to test someone with eagle eyes at a distance further than 10 meters up to 30 meters. Surely the device has better to be a tablet or iPad.

Show Optotype Height

VAC delivers optically precise optotype according to the design principles of Snellen Chart and its variants. If you want to double-check, you may turn this on and use an accurate ruler to check the heights of optotypes at different test distances. Or, you may use this to verify other app or paper based visual acuity charts.

Play Interval Until Next Optotype

During Auto-scoring, the app will display next optotype after the defined interval even if the doctor doesn't indicate whether the subject is reading correctly. And this will mark a miss if the “patient” could not read correctly on time within the interval. 

Record Scores

During Auto-scoring, the app will display the test result of one eye. And when this setting is on, you may save the result.

VAC saves both result of non-VAS notation and the result of VAS notation. Then you may see the progress of visual acuity of a subject overtime.

For young children, showing them the progression, especially with the decrement of visual acuity may motivate them to take more precautious actions and play more outdoor activities during the day. It is highly possible to reverse the progression of pseudomyopia if finding the decrement of visual acuity early and act ASAP. 

And you can share the records with your doctor in telemedicine or with your children.

Multiple Optotypes

Having multiple optotypes in one row is handy to spot lazy eye (Amblyopia) which may take a few tests by medical practitioners to diagnose. 


  • Please notes, Snellen Chart and its variants are not adequate for the detection and diagnosis of lazy eye. Please do make an appointment with pediatricians or optometrists ASAP upon preliminary sign of amblyopia.

Tumbling E with Short Middle Arm

This setting is available on when current chart is Tumbling E.



  • The setting takes effect after you switch to the other chart then switch back to Tumbling E.