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.natural eye 1 image by Melanie von Snarly from Fotolia.com
  
  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.lens image by Stanisa Martinovic from Fotolia.com
  
  The lenses in cameras, camcorders and telescopes are designed to work like the human eye, "dilating" or "constricting" to permit more or less light.