Visual acuity is the ability to see and discriminate a certain size target at a specified distance. Distance vision can be measured by various methods, but basically looks like a fraction, e.g. 20/20, 20/200. The first figure (numerator) records the distance at which the testing was done (in feet). The smallest line on the chart that the patient identifies at this distance is observed. The second figure (denominator) gives the comparative distance (in feet) at which a normal sighted individual would identify this line. For example, a person with 20/100 vision sees at 20 feet what a person with normal vision sees at 100 feet.
In patients with impaired vision, we use a Feinbloom chart (or similar high contrast test cards) instead of the standard Snellen projection chart at a 20 ft distance. The advantages of testing with a Feinbloom chart are:
Since these test materials are designed to provide optimal conditions for the patient, we can get better visual acuities for a patient by using the Feinbloom chart than by the Snellen chart, as the table below shows:
|Hand-held Feinbloom chart||Snellen projection chart|
|OD 5/100 = approximately 20/400||OD unable to identify target|
|OS 10/100 = approximately 20/200||OS 20/400|
|OU 10/120 = approximately 20/240||OU 20/400|
OD = right eye
OS = left eye
OU = both eyes
There are many charts for assessing near acuity. The metric, or M system is a linear scale of reading visual acuities, unlike the Jaeger or point notation, and is easily tested and recorded at any distance. 1 M letters are 1.45 mm tall and are roughly equivalent to magazine print. Similarly, 2 M letters are 1.95 mm tall, about the size of newspaper subheadlines. Remember that identifying letters and comfortably reading print are often several lines different, the latter requiring larger-sized letters.
The patient's near vision is tested by using lines of text printed in high contrast and viewed at an appropriate reading distance using optimal illumination. The smallest line accurately read is recorded. In real life, reading is undertaken with text of varying quality and contrast, such as newspaper print, and the patient may not be able to read as well as suggested by the standardized near vision testing. Therefore, it is necessary to evaluate the use of the recommended glasses prescription in a trial frame with the patient's desired reading material. To achieve optimal reading, environmental lighting modifications and use of color filters to enhance contrast may also be necessary.
Using a Lighthouse adult continuous text card:
OD: unable to identify target
OS: 4.0 M @ 30 cm
OU: 4.0 M @ 30 cm
This card represents the size of text that could be read at his chosen distance of 30 cm (13 inches). The 4.0 M notation is equivalent to 5.0 mm letters or slightly under 2" tall lower case print.
In addition to text size, several other factors should be considered in determining the patient's ability to function at near vision tasks: