Acuity - Visual Acuity is defined as the acuteness or clearness of vision. It is dependent on the sharpness of the focus of the retina in the eye and the ability of the brain to process the information.

Acuity is measured in numbers. A “normal” visual acuity is 20/20. The top number is the distance (in feet) someone needs to stand from an object in order to see it clearly. The bottom number is the distance (in feet) that a person with “normal” vision can reliably distinguish between objects separated by an angle of 1 arc minute (1/60 of one degree).

An eye exam seeks to find the prescription that will provide the individual with 20/20 vision. Each eye can have its own acuity for both distance and near vision as well as acuity for both eyes when they are used together.

Anatomy of the Eye

Anterior Chamber - The anterior chamber is a space between the cornea and the iris. It is filled with a clear nutrient fluid called the aqueous humor. A build up of fluid in the anterior chamber can cause glaucoma.

Conjunctiva - The conjunctiva covers the outer surface of the sclera and lines the eyelids. It is a thin, transparent tissue that begins at the outer edges of the cornea and has tiny blood vessels that nourish it. It also secretes mucous and oil to moisten and lubricate the eye.

Cornea - The cornea is a clear layer of the eye that covers the iris and pupil. It is only half of a millimeter thick, but provides two-thirds of the focusing power of the eye. Although it has no blood vesicles, it has more nerve endings than any other area of the human body and is therefore extremely sensitive.

Iris - The section of the eye that contains the color is called the iris. The iris works like the lens of a camera. Muscles open to allow more light and close to allow less light. The iris is the dividing point between the front and back of the eye. The color, texture, and patterns of each person's iris are as unique as a fingerprint.

Lens - The lens is located immediately behind the iris. The lens is used to focus light on the retina and is held in place by tiny structures called zonules. The lens changes shape to adjust between distance and near vision. When an individual is young, the iris has a jelly-like consistency. As people age, the lens becomes harder which makes it more difficult for individuals to focus on an object.

Macula - The macula is located roughly in the center of the retina, next to the optic nerve. The macula is the most sensitive part of the retina and contains the cones. It is responsible for seeing detail and central vision. The fovea is the very center of the macula and contains the majority of the cones.

Optic Nerve - The optic nerve transmits electrical impulses from the retina to the brain. It is connected to the back of the eye near the macula. The connection point is called the optic disc and can be seen during an eye exam. Because the optic disc contains no rods or cones, this area is a blind spot. The blind spot is not normally noticeable because the combined vision of both eyes overlaps.

Pupil - The pupil is actually a hole in the center of the iris. The size of the pupil depends on the iris. As more light is needed, the iris expands and the pupil gets larger. As less light is needed, the iris contracts and the pupil gets smaller. The pupil appears to be black due to the lack of light in the rear section of the eye.

Retina - The retina lines the back of the eye. It is a multi-layered tissue that contains photoreceptors which capture light rays and convert them to electrical impulses. These impulses then travel through the optic nerve to the brain where they are turned into images. The two types of photoreceptors contained in the retina are rods and cones. There are approximately 125 million rods which are located in the peripheral area of the retina. The rods are responsible for peripheral vision and night vision. They function best in dim lighting. There are approximately 6 million cones. The cones are located in the macula and are responsible for color vision. They function best in bright light.

Sclera - The “white part” of the eye is called the sclera. It is a very tough tissue that serves as protection for the eye. When an individual is young, the white takes on a blue color due to it being thin. As the individual ages it tends to become more yellow. The sclera is attached to six muscles which control the movement of the eye. The optic nerve is attached to the sclera at the very back of the eye.

Vitreous - The vitreous is a transparent, liquid substance that allows the eye to maintain its shape. It is located in the back section of the eye and is attached to the retina in several areas. When an individual is young, the vitreous is similar to the consistency of an egg white. As people age, the vitreous becomes thinner and separates from the retina. This causes “floaters” which can cast a shadow on the retina.