How Vision Works: The Eye, Accommodation, and Night Vision

Vision is the joint work of the eye, the optic nerve, and areas of the brain that together create an image of the surrounding world. Below is a step-by-step breakdown of how light passes through the eye, how the eye focuses on objects at different distances, and why a person sees differently in the dark than during the day.

How vision works: the path of light from the eye to the brain

The visual system is the set of organs and neural structures that perceive visible light and form visual images, making it possible to distinguish shape, color, movement, and the position of objects. The stimulus for the organ of vision is light: under its influence, photochemical changes in visual pigments occur in the rods, cones, and light-sensitive ganglion cells of the retina.

From the retina, the signal travels along the optic nerves, then through their crossing, called the chiasm, and along the optic tract to the lateral geniculate body — a subcortical center of the diencephalon — and from there to the visual cortex. The visual fields of the two eyes overlap, the signals are compared, and the person gains three-dimensional, binocular vision.

Anatomy of the eye: main structures

The eyeball lies in the orbit — a paired bony cavity of the skull. The eye is moved by six external muscles: four rectus and two oblique.

Light is perceived by the retina with its photoreceptors.

Accommodation: how the eye focuses

For the image to be clear, light rays from an object must converge precisely on the retina. This task is handled by the lens — a transparent elastic lens inside the eye: its curvature is changed by muscles, and the eye focuses on distant or near objects. This adjustment of focus is called accommodation: when looking at a near object, the lens becomes more convex; when looking into the distance, it becomes flatter.

If the refractive power of the eye does not match its length, the focus moves away from the retina and vision becomes blurred. In that case, glasses are selected during an examination using a device with lenses of different powers.

Night and twilight vision

Two types of photoreceptors work in the retina. Rods function in low light, and the sensations they produce are colorless. Cones function in daylight and bright light and provide color vision. That is why at dusk a person mainly distinguishes outlines, and colors appear muted.

Difficulty seeing at night and in low light is among the signs of deteriorating vision — along with difficulty reading, watching television, and recognizing faces, and discomfort from bright light. Vision deterioration can occur at any age, but is more common with advancing years.

How vision is examined and what is considered normal

Visual acuity, that is, the ability to distinguish fine details, is examined using a chart: the person reads symbols from a distance of about twenty feet, roughly six meters. The doctor evaluates the result together with the data from the rest of the examination and explains what it means.

A more complete examination with pupil dilation includes several checks.

• Visual field testing, that is, peripheral vision: its loss may be a sign of glaucoma.
• Testing of the eye muscles: the person follows an object with their gaze.
• Testing of the pupils' reaction to light.
• Measurement of the pressure inside the eye, or tonometry: helps detect glaucoma.

The pupil is then dilated with drops: more light enters the eye, and the doctor examines the retina, macula, and optic nerve through a magnifying lens.

When changes in vision require an ophthalmologist's examination

You should consult an optician or an ophthalmologist if you suspect deteriorating vision: early treatment helps stop the deterioration. Blurred and cloudy vision, a glow around light sources, and less vivid colors occur with cataracts. Spots, lines, flashes, darkness, or shadows, often sudden, occur with retinal detachment. Distortion of the center, difficulty reading and recognizing faces occur with age-related macular degeneration. Gradual deterioration in diabetes occurs with diabetic retinopathy. Blurred vision with severe eye pain and redness occurs with glaucoma.

An urgent appointment is needed for changes in vision — blurring, double vision, flashes of light or shapes; for a dark shadow moving in front of the eye; for pain in one or both eyes; for a red and painful eye; for painful sensations from bright light.

Difficulty seeing at dusk on its own is not a reason to call an ambulance. The numbers 103 or 112 are needed if vision suddenly disappears in one or both eyes, as well as for sudden severe eye pain.

Vision is the coordinated work of the eye and the brain. This material is for reference and does not replace a doctor's consultation.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer