FAQ
What do these charts measure?
These are screening charts for close up and distance visual acuity. Visual acuity (see "What is visual acuity") is only a part of eye examination. If you are not an appropriately qualified health professional you should not rely on these tests for diagnosis of conditions affecting vision. There are many serious medical conditions in which visual acuity is not affected or only slightly affected.
What is visual acuity?
Wikipedia has a good article explaining visual acuity.
Why measure visual acuity?
VA is measured for many reasons, including,
- statutary requirements - for instance driver and aircraft pilot licensing
- for health screening - for example in school children
- as part of visual assessment for diagnosis and management of vision disorders
- for research.
What is an N chart?
An N Chart is a set of texts of various sizes used for assessment of near vision. The number after the N refers to the size of the type in printers' points. A printers' point is 1/72 inch. The point size is not necessarily the height of individual letters in the type.
Why does the app need to be calibrated?
The displayed size of the test letters ("optotypes") is critical to the accuracy of the tests.
The number of iPad models is constantly increasing. iPads have differing pixel densities, and there is currently no way that an app can reliably determine these programmatically. The only reliable way to ensure that the displayed size is correct is to physically measure a line on the screen and enter this value so that the app can calculate the correct scale.
This calibration only needs to be done once after the app is installed. It only needs to be repeated if the app is reinstalled or if it is installed on another iPad.
How do I know that I have calibrated the app correctly and that the letters are displaying at the correct size?
The 6/60 letters should be 87.3mm tall, and the 20/200 letters should be 88.6mm tall. (20 feet is slightly further than 6 metres.)
It is strongly recommended that you check that this is the case after you have calibrated the app and before you use it for testing.
Why can’t the distance be set to "X" meters / feet? I don’t have a 6 metre room.
Distance vision needs to be done at, well, distance. If the test subject is closer to the chart, near vision starts to be measured. Short sighted people with score better and long sighted people worse. If your room is at least 3 metres (or 10 feet) long, then you can have the subject read the letters in a mirror placed on one wall, and display the Far chart with letters reversed, by swiping right or left on them.
Why does the letter size change when I change from 6/60 to 20/200?
Twenty feet is slightly more than six metres. The letter size changes slightly to maintain the same angular size of the letter. This is fifty arc minutes at 6/60 and 20/200, and five arc minutes at 6/6 and 20/20.
Can you add a set of Ishihara colour blindness plates to the app?
We regret that we are unable to incorporate this function in the app, as the Ishihara plates are under international copyright by the Isshinkai Foundation, Tokyo, Japan. In addition to copyright restrictions, colour blindness testing is highly dependent on standardised colour rendering, and due to the diversity of devices available, it is almost impossible to guarantee that the results would be accurate.
Is there an iPhone version?
No. iPhone screens do not display enough letters at the required size at the same time.
Is there an Android version?
No. Near and Far is an iPad only app. It is not available for other devices or operating systems.