Why the Eye Doctor Dilates Your Eyes

It's all about how light passes through

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Your eye doctor may temporarily dilate your eyes with eye drops to widen your pupil (black part of your eye) so that they can see the structures of your eye. This helps with your vision assessment so you can get the right eyeglass or contact lens prescription.

Most vision problems are caused by a refractive error—a defect in the eye that affects how light bends when it passes through. Shining a light into your eyes while they're dilated can help your eye doctor spot and measure refractive errors, as well as identify other potential problems.

This article reviews the purpose of dilation, what conditions it helps diagnose and treat, when you're more likely to need it done, how long it lasts, and what the side effects can be.

Dilated eye

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Purpose

In a healthy eye, light hits the cornea (outer layer) and angles to the lens inside your eye. The lens focuses the light onto your retina (tissue at the back of your eye).

Via the optic nerve, the retina sends signals to the brain about the patterns of light that you see. Your brain interprets those patterns and tells you what you're looking at.

With a refractive error, light bends at a different angle. A dilated eye exam allows the doctor to measure the degree of light refraction. Another aspect of dilation is that it may help determine your true refractive error because it stops your eye from focusing, This can be helpful for your corrective lens prescription.

The dilating drops:

  • Make the pupil wider, which gives the eye doctor a bigger opening to look though
  • Allow more light to pass through and to be refracted by the cornea
  • Paralyze muscles that help you focus to keep you from subconsciously focusing your eyes, which can affect vision measurement results

The most common vision problems involve refractive errors:

Because dilation allows the doctor to see more parts of your eye, including blood vessels, it also allows them to check for other common eye problems, such as:

In some cases, certain dilating drops are used as a treatment. These are generally longer-lasting drops that can lower pain and inflammation from eye disease, surgery, or injury. They're also used to treat "lazy eye" (amblyopia) and a progressive form of myopia.

When It's Needed

If you're seeing an eye doctor for the first time, expect your eyes to be dilated. You will need your eyes dilated at every yearly comprehensive exam. If you have diabetes, macular degeneration, or glaucoma, you may need a dilated eye exam more often than once a year.

You may need also more frequent eye exams if you're:

  • Over age 40 and Black or Hispanic
  • Over age 60 and any other ethnicity
  • Have a family history of glaucoma

To determine how often you need a dilated eye exam, your doctor will also consider your overall health, eye health history, and whether you've had abnormal findings in earlier dilated exams.

For some eye exams, like if you are being seen for blepharitis (eyelid inflammation), pink eye, or follow up for contact lenses, you might not need to have your eyes dilated.

Children​

Children can be hard to examine. They're more likely to focus automatically and are usually less able to control eye movements than adults. They also may not be able to switch focus from close to distant objects when asked.

Because of these potential problems, and because they are growing and changing, kids may need to have their eyes dilated at every exam so the doctor can properly look at their eyes.

For Adults

Presbyopia is an inability to focus up close. It affects almost everyone older than 40. When it develops before that, it's called premature or pre-presbyopia.

Pre-presbyopia makes you unable to:

  • Quickly change focus from near to far
  • Focus properly at night

Your doctor would need to see the inside of your eye to diagnose pre-presbyopia.

LASIK Candidates

You need a dilated eye exam before laser vision correction (LASIK) or other vision correction surgery. Your surgeon would use this test to determine exactly how much to change the shape of your cornea to correct the refraction error.

Recap

Dilation is done to assess refractive errors, help diagnose conditions like retinal detachment, and determine candidacy for vision correction surgery.

Dilation Timeline

The amount of time it takes for your eyes to dilate and to stay dilated dilation depends on the drops used and your sensitivity to the drops. There are different drops used for different purposes.

It often will take between 20 and 30 minutes after the drops are put in for your eyes to be fully dilated. The dilation typically stays in effect for between two and eight hours, though it may last 24 hours or even longer for some people.

The color of your eyes also affects this, as pigment binds with the drops and changes how they work.

Brown eyes, which have a lot of pigment, need stronger doses and more time to dilate, and the recovery is on the shorter side. Hazel, green, and blue eyes have less pigment and require lower doses, dilate faster, and recover more slowly.

Children also need stronger drops than adults, and their dilation may last longer.

Side Effects

Dilating eye drops may sting for a few seconds. Some of them cause blurriness and light sensitivity for several hours. Occasionally, this will even last overnight.

Eye doctors generally give out disposable sunglasses for when you leave the office with your eyes dilated. You may want to bring your own so you're sure you have something that fits well.

Some people who have glaucoma can experience a jump in their eye pressure when dilated. Pressure-lowering drops prior to dilation may temper this effect. In some cases, dilation may not be advised.

Recap

Your vision will be blurry and sensitive to light after dilation. This may last a few hours or, if you have light-colored eyes, as long as a day.

After Dilation

It is not safe to drive after your dilated exam, so be sure to ask if you are set to be dilated and arrange for a ride home if so.

Other activities may also be unsafe or difficult while you are waiting for the drops to wear off. You may want to consider taking some time off or making your eye appointment late in the afternoon, evening, or weekend if it's available so you don't miss a lot of work.

Check with your eye doctor to see if it's OK for you to put in your contact lenses while your eyes are dilated.

Summary

Dilating eye drops help eye doctors diagnose refractive errors, other vision problems, and eye health conditions. They widen your pupil, let in more light, and paralyze eye muscles.

You shouldn't need dilation at every exam, but certain tests and procedures require it. The temporary light sensitivity and blurred vision that result from dilation make driving and some other activities unsafe until the drops wear off, so it's important that you plan ahead for this.

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5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. New York University, Langone Health. Types of refractive error.

  2. American Association for Pediatric Ophthalmology and Strabismus. Dilating eye drops. Updated April 2020.

  3. University of Utah, U Health. Is it necessary to have your eyes dilated during every eye exam? Updated March 5, 2014.

  4. Anderson HA, Bertrand KC, Manny RE, Hu YS, Fern KD. Comparison of two drug combinations for dilating dark iridesOptom Vis Sci. 2010;87(2):120-124. doi:10.1097/OPX.0b013e3181cc8da3

  5. Whitman J. American Academy of Ophthalmology. Should patients with severe glaucoma allow their eyes to be dilated during exam? April 24, 2019.

Additional Reading
  • National Institutes of Health, National Eye Institute. Refractive errors. Updated August 28, 2020.

  • National Institutes of Health, National Eye Institute. Get a dilated eye exam. Updated May 19, 2021.