Eye anatomy and Structure
How the many components of the eye work in concert to produce clear vision.
Key points
The definition of visual acuity (VA) is that of the clarity of the image visible to the eye. Visual acuity is measured by looking at an eye chart from a distance of twenty feet.
It is important to note that 20/20 vision is not perfect vision. It refers to normal, average vision.
VF stands for visual field, and it describes the distance you can see to the sides. A normal visual field is 180 degrees, which is a half-circle.
It is important to note that the term “legally blind” means different things in different provinces and states. Find out whether your child qualifies for an assistive device program, if he or she is able to drive, or if he or she qualifies for other benefits by consulting with your optometrist or ophthalmologist.
The eye anatomy
There are many components of the eye that must work together in order to produce clear vision:
Sclera is the white portion of the eyeball that protects it.
Located in the center of the eye, the pupil is a black dot that allows light to enter the eye.
It is the iris which controls the amount of light entering the eye by controlling the size of the pupil.
The cornea is a transparent window that covers the iris and pupil of the eye.
By focusing light onto the retina at the back of the eye, a clear lens, located behind the pupil, acts like a camera lens.
A retina is a light-sensitive inner layer that lies at the back of the eye. Ten layers of cells work together to detect light and convert it into electrical impulses.
The retina
The retina consists of special cells known as cones and rods that serve as photoreceptors and absorb light.
Cones
Approximately half of the cone cells are located in the macula, which is the central part of the retina. Cones provide us with the ability to see color and detail. The macula also permits us to read and recognize facial details, such as eye color and freckles, with great precision.
Rods
Our night vision is derived from rod cells located in the peripheral area of the retina. Rod cells enable us to see in low-light conditions and provide us with the ability to see in the dark.
How the eye sees
The following sequence occurs in people with normally functioning eyes:
The object we are looking at reflects light.
At the front of the eye, the cornea receives light rays.
To reach the lens, light passes through a watery fluid (aqueous humor).
As the lens changes in thickness, it bends light, which focuses it onto the retina at the back of the eye.
Vitreous humor is a thick, clear fluid that fills the eyeball and helps maintain its round shape along the way to the retina.
After reaching the back of the eye, the light is translated to electrical impulses by the retina, which are then transmitted to the brain by the optic nerve.
Last but not least, the visual cortex (or visual center) of the brain is responsible for interpreting these impulses into visual images.
What normal vision is like
It is helpful to have a basic understanding of normal vision in order to comprehend the vision of someone with an eye condition.
Assume that two people are sitting on the couch in front of you. If you look directly at Person A, you will be able to see detailed features of their head and face, such as freckles, brown eyes, and black hair.
Additionally, you are aware that Person B is sitting beside Person A on the couch. However, you are unable to observe the same level of detail on their face. For example, you may only be able to see dark areas near their eyes. To see Person B, you are utilizing your peripheral vision, the remaining part of your retina. Normal vision is defined as seeing clearly and sharply in the center, but blurry in the periphery.
Vision problems
There are many types of eye conditions that can affect vision in a variety of ways, including problems with any part of the eye. Occasionally, the lens is not focused properly or the eyeball is not round, resulting in blurry images. This can often be corrected with prescription lenses or contact lenses. Farsightedness is defined as an image that is focused behind the retina. Nearsightedness (myopia) occurs when an image is focused in front of the retina.
Eye conditions that affect the retina include those affecting only their peripheral vision, which can cause tunnel vision in some individuals, while in others, the condition affects only their central vision, causing blind spots (scotomas) to appear. In addition to cataracts, glaucoma, corneal damage, or muscle problems are other eye conditions that can result in vision problems.
Visual field (VF)
The visual field (VF) is the term used to describe the width of one’s peripheral vision. Normally, our visual field is 180 degrees or half a circle. When looking straight ahead, we should be able to detect movement off our right or left shoulders.
How VF is measured
VF can be measured in a variety of ways. The most common way is using an automated machine, such as the Humphrey Visual Field Analyzer. However, a manual test, such as the Goldmann Visual Field Test, may also be used.
People are asked to focus their eyes straight forward in order to measure their VF. Next, a light source is introduced into their field of vision from the side. When the person sees the light, they push a button. The results of the VF test determine whether an individual has tunnel vision and blind spots.
There are two ways to measure the visual fields: one eye at a time or both eyes at the same time. In order to ensure that one eye sees in an area which is a blind spot in the other eye, a person with blind spots in both eyes can undergo a binocular visual field test. As a result, it is critical to determine whether or not a person has a VF that meets the government’s standards for driving.
Visual acuity (VA)
An eye chart is used to measure visual acuity (VA). At a distance of 20 feet (6 meters), the eye chart is used to test visual acuity. The average vision of most people has been determined by testing many patients when standing 20 feet away from a chart of eyes. This measurement is called “normal” vision.
Children who are unable to read the letters on the chart will have their vision tested using preferential looking tests, which involve holding up cards with lines or pictures. In addition to looking to the side of the card with the picture (left or right, up or down), the child will usually indicate that they can see the card without verbally responding. During the test, the cards become increasingly difficult to see, and the test continues until a child stops responding. As a result of the grading of the cards, it is possible to estimate a person’s vision level on the 20/20 scale.
The VA of 20/20 signifies that a person with normal vision can see clearly line 20 when standing at a distance of 20 feet from the eye chart. In metric units, this is known as 6/6 VA since 20 feet equals six metres.
If corrective lenses cannot correct vision to at least 20/50, or worse, a person with poor VA may not be able to drive a car. If corrective lenses cannot correct vision to 20/20, then the individual’s vision is considered too blurry to drive.
Examples of visual acuity
When a person has 20/50 vision, he or she will be able to see what a 20/20 sighted individual is able to see when standing 50 feet away. This implies that a person with 20/50 vision will have to stand 30 feet closer to the eye chart than a 20/20 individual to see the same detail.
The biggest letter on the eye chart (usually the letter E) can be read by someone with a VA of 20/200, but all other lines on the chart are blurry. Those with a VA of 20/200 can see as much detail as a 20/20 sighted individual from 200 feet away by being located 20 feet from the eye chart.
Having 20/20 vision does not necessarily mean that you have perfect vision. In fact, it may only indicate that you have average vision. There are other vision skills that can affect your overall visual ability, which are:
Awareness of the peripheral environment or side vision
Coordination of the eyes
Perception of depth
Focusing ability
Visual perception of color
Definitions of “legally blind”
There is no standard definition of “legally blind” in every province, state, or country. You should ask your optometrist or ophthalmologist for the definition in your area.
Driving
A person must have a VA of at least 20/50 when driving with a private G class license in Ontario, and both eyes must be open with corrective lenses. The VF of the individual must be at least 120 degrees, with neither blind spots nor interruptions of vision. Both eyes must also be open at a distance of at least 15 degrees above and below the fixation point.
If a patient’s VA or VF does not meet the guidelines set by the Ministry of Transportation (MOT), their vision may be classified as “legally blind” for driving purposes. Ophthalmologists are legally required to report patients who do not meet the MOT guidelines.
Assistive Devices Program (ADP)
It is necessary for a person to have a VA of 20/70 or worse with their best correction to qualify for low vision aid funding through the Assistive Devices Program (ADP). Those individuals classified as “legally blind for assistive devices” should consult with a low vision specialist, or a licensed optometrist, to determine which low vision aids are available and appropriate for their vision problems. Magnifying hand-held devices, reading glasses with magnification, and telescopes are some of the low vision aids available.
