Distance & Near Vision

Visual acuity

Visual acuity is the clarity of your eyesight, measured by your ability to identify letters or numbers on a standardised eye chart from a specific viewing distance.

Visual acuity is a static measurement, meaning you are sitting still during the testing and the letters or numbers you are viewing also are stationary.

Visual acuity also is tested under high contrast conditions — typically, the letters or numbers on the eye chart are black, and the background of the chart is white.

Although visual acuity testing is very useful to determine the relative clarity of your eyesight in standardised conditions, it isn't predictive of the quality of your vision in all situations.

Three major physical and neurological factors determine visual acuity:

How accurately the cornea and lens of the eye focus light onto the retina
The sensitivity of the nerves in the retina and vision centres in the brain
The ability of the brain to interpret information received from the eyes

Visual acuity refers to a person's ability to see small details. It is also referred to as clarity of vision or sharpness of vision. Visual acuity is dependent on a combination of optical and neural elements, including:

The health and functionality of the retina
How sharply the retina forms images
How well your brain can interpret sensory inputs

Refractive errors or neural factors usually cause poor visual acuity.

Refractive errors are visual impairments that affect the way light is bent, or refracted, in your eyeball.

Common refractive errors include:

Hyperopia (farsightedness)
Myopia (nearsightedness)
Astigmatism (irregular curvature of the cornea)

Neural factors occur in the retina, brain, or the pathway between the two.

Examples include:

Age-related macular degeneration (AMD)
Detached retina
Amblyopia (lazy eye)
Brain injuries
Stroke

Most of the time, lower visual acuity can be corrected by eyeglasses, contact lenses, or refractive surgeries.

Distance Vision

Eye charts are the most common method of visual acuity testing. They show several rows of optotypes (letters, numbers, or symbols) that get progressively smaller towards the bottom.

How is a Visual Acuity Test Performed?

During the eye test, the client will be seated/stood a certain distance from the visual acuity chart. Elements such as distance from the test chart and lighting conditions must be standardised.

You'll cover Their right or left eye and ask them to read the chart starting at the top and proceeding downwards until they can no longer distinguish the letters.

Then you'll switch eyes and repeat the process from the same viewing distance.

How to Read the Results of a Visual Acuity Test

Visual acuity is typically measured in fractions or decimals.

The first number in the fraction refers to the testing distance, and the second number refers to the distance someone with "normal" vision could see the same details from.

For example, 20/20 vision means that a person can see an object as well as anyone with "normal" vision from 20 feet.

If they have 20/30 vision (a lower visual acuity score), it means the details they see from 20 feet away can be seen from 30 feet away by someone with "normal" visual abilities.

Basically, the higher the second fraction number, the lower the visual function.

In most other countries, visual acuity is expressed using the metric system. Because 20 feet is equivalent to 6.096 meters, 20/20 vision is equal to 6/6 vision.

Near Vision

How is near vision tested?

A near-vision chart usually has test types based on the printer’s ‘N’ series. The smallest is numbered N5 and successive numbers indicate larger types. The usual distance for gauging near reading, till the age of 40 years, is 14 inches.

Symptoms of nearsightedness include:

Blurred vision
Headaches
Eye strain
Squinting of eyes to see clearly
Excessive blinking
Rubbing eyes in children

Causes

The causes include:

Caused when the cornea or lens is too curved for the length of the eyeball
The shape of the eye is known to be inherited
The risk factors include:
Family history of nearsightedness
A lot of reading and spending most of the time indoors

The Jaeger eye chart (or Jaeger card) is used to test and document near visual acuity at a normal reading distance. Refractive errors and conditions that cause blurry reading vision include astigmatism, hyperopia (farsightedness) and presbyopia (loss of near focusing ability after age 40). If you typically wear eyeglasses or contact lenses full-time, you should wear them during the test.

Hold the test card 14 inches from the eyes. Use a tape measure to verify this distance.
The card should be illuminated with lighting typical of that used for comfortable reading.
Testing usually is performed with both eyes open; but if a significant difference between the two eyes is suspected, cover one eye and test each eye separately..
Go to the smallest block of text you feel you can see without squinting, and read that passage aloud.
Then try reading the next smaller block of text. (Remember: no squinting!)
Continue reading successively smaller blocks of print until you reach a size that is not legible.
Record the “J” value of the smallest block of text you can read (example: “J1”).

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