What Is Eye Dilation?

The eye dilates or constricts in response to some stimulus---chiefly, light.

When the pupil (the dark center of the eyeball) widens, this is called dilation, or mydriasis. Constriction, also called miosis, is the opposite effect---the narrowing of the pupil. Several factors, both normal and unhealthy, cause these responses.

Light

Dilation allows in more light and constriction allows in less. The musculature in the iris (the colored portion of your eyeball) dilates or constricts as needed. Thus, as light dims, your pupils dilate, and as it brightens, your pupils constrict.

Examinations

As the Mayo Clinic describes, dilation with eyedrops during an exam "can help your doctor diagnose ... diseases and conditions, such as: diabetes; eye tumors; high blood pressure; infectious diseases; macular degeneration; retinal detachment; vasculitis."

Emotions

Emotionally stimulated dilation varies from one person to the next.

Fear and sexual arousal can cause pupils to dilate. Individual responses to stimuli like food and anger also are possible causes.

Disease and Injury

Unusual dilation of the pupils is a symptom, not a disease by itself. Some causes include cataracts; drug use (for example, of methamphetamines); and damage to the ocular nerve or head injury, which can cause nonparallel (uneven) dilation of the two eyes.

Optics

A camera's lens is modeled after the human eye.

The lenses in cameras, camcorders and telescopes are designed to work like the human eye, "dilating" or "constricting" to permit more or less light.

References

• The Mayo Clinic
• "Clinical Anatomy of the Eye;" Richard Snell and Michelle Lemp; 1998