What structure regulates the amount of light passing to the retina?

To understand how certain problems can affect your child's vision, it’s important to know how normal vision happens. For children with normal vision, the following things happen in this order:

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How the Eye Works
How and what regulates light passing through the eye?
What determines how much light reaches the retina?
What structure changes focus to light on the retina?

Light enters the eye through the cornea. This is the clear, dome-shaped surface that covers the front of the eye.

From the cornea, the light passes through the pupil. The iris, or the colored part of your eye, controls the amount of light passing through.

From there, it then hits the lens. This is the clear structure inside the eye that focuses light rays onto the retina.

Next, light passes through the vitreous humor. This is the clear, jelly-like substance that fills the center of the eye. It helps to keep the eye round in shape.

Finally, the light reaches the retina. This is the light-sensitive nerve layer that lines the back of the eye. Here the image is inverted.

The optic nerve is then responsible for carrying the signals to the visual cortex of the brain. The visual cortex turns the signals into images (for example, our vision).

Choroid
Layer containing blood vessels that lines the back of the eye and is located between the retina (the inner light-sensitive layer) and the sclera (the outer white eye wall).
Ciliary Body
Structure containing muscle and is located behind the iris, which focuses the lens.
Cornea
The clear front window of the eye which transmits and focuses (i.e., sharpness or clarity) light into the eye. Corrective laser surgery reshapes the cornea, changing the focus.
Fovea
The center of the macula which provides the sharp vision.
Iris
The colored part of the eye which helps regulate the amount of light entering the eye. When there is bright light, the iris closes the pupil to let in less light. And when there is low light, the iris opens up the pupil to let in more light.
Lens
Focuses light rays onto the retina. The lens is transparent, and can be replaced if necessary. Our lens deteriorates as we age, resulting in the need for reading glasses. Intraocular lenses are used to replace lenses clouded by cataracts.
Macula
The area in the retina that contains special light-sensitive cells. In the macula these light-sensitive cells allow us to see fine details clearly in the center of our visual field. The deterioration of the macula is a common condition as we get older (age related macular degeneration or ARMD).
Optic Nerve
A bundle of more than a million nerve fibers carrying visual messages from the retina to the brain. (In order to see, we must have light and our eyes must be connected to the brain.) Your brain actually controls what you see, since it combines images. The retina sees images upside down but the brain turns images right side up. This reversal of the images that we see is much like a mirror in a camera. Glaucoma is one of the most common eye conditions related to optic nerve damage.
Pupil
The dark center opening in the middle of the iris. The pupil changes size to adjust for the amount of light available (smaller for bright light and larger for low light). This opening and closing of light into the eye is much like the aperture in most 35 mm cameras which lets in more or less light depending upon the conditions.
Retina
The nerve layer lining the back of the eye. The retina senses light and creates electrical impulses that are sent through the optic nerve to the brain.
Sclera
The white outer coat of the eye, surrounding the iris.
Vitreous Humor
The, clear, gelatinous substance filling the central cavity of the eye.

How the Eye Works

The five senses include sight, sound, taste, hearing and touch. Sight, like the other senses is closely related to other parts of our anatomy. The eye is connected to the brain and dependent upon the brain to interpret what we see.

How we see depends upon the transfer of light. Light passes through the front of the eye (cornea) to the lens. The cornea and the lens help to focus the light rays onto the back of the eye (retina). The cells in the retina absorb and convert the light to electrochemical impulses which are transferred along the optic nerve and then to the brain.

The eye works much the same as a camera. The shutter of a camera can close or open depending upon the amount of light needed to expose the film in the back of the camera. The eye, like the camera shutter, operates in the same way. The iris and the pupil control how much light to let into the back of the eye. When it is very dark, our pupils are very large, letting in more light. The lens of a camera is able to focus on objects far away and up close with the help of mirrors and other mechanical devices. The lens of the eye helps us to focus but sometimes needs some additional help in order to focus clearly. Glasses, contact lenses, and artificial lenses all help us to see more clearly.

How and what regulates light passing through the eye?

The iris (the colored part of the eye) controls how much light the pupil lets in. Next, light passes through the lens (a clear inner part of the eye). The lens works together with the cornea to focus light correctly on the retina.

What determines how much light reaches the retina?

There are three major prereceptoral factors governing the amount of light reaching the retina: pupil area, lens optical density and the optical density of the macular pigment. Standardized tabulations are available for each of these factors.

What structure changes focus to light on the retina?

Behind the iris sits the lens. By changing its shape, the lens focuses light onto the retina. Through the action of small muscles (called the ciliary muscles), the lens becomes thicker to focus on nearby objects and thinner to focus on distant objects.