LASIK surgeries are performed via laser, and the U.S. Food and Drug Administration sets treatment parameters for surgery. Per those guidelines, people with up to +6 diopters of hyperopia or up to -12 diopters of myopia can have LASIK correction.

LASIK surgeries do come with risks, and doctors want to ensure that the potential benefits make taking those risks worthwhile. For some doctors, a stronger prescription means a bigger chance you will be happy with the outcome after surgery. But there are times when surgery is not right, as your prescription is too high.

Before LASIK, your doctor assesses your vision, and the tests are likely to be more intense than those you are accustomed to in a regular eye exam. Your doctor must also evaluate your eye health to ensure that you are healthy enough for surgery.

Hundreds of thousands of people get LASIK surgery every year. If you are one of them, your doctor will work hard to ensure that your vision is crisp and clear after surgery.

Formal Vision Guidelines

To qualify for LASIK, you must have a stable prescription. That means your prescription for glasses or contacts must stay the same for a year or so before the surgery starts. Young people often have shifting prescriptions as they grow, but adults can have changing prescriptions due to pregnancy, breastfeeding, and steroid drugs.

A stable prescription is not the only requirement for LASIK. You will also need numbers that fit within guidelines specified by the U.S. Food and Drug Administration. These parameters are set per device, meaning that some lasers are stronger than others and can tackle procedures accordingly.

But in general, experts say, these are the limits:

  • Nearsightedness: Up to +6 diopters
  • Farsightedness: Up to -12 diopters
  • Astigmatism: Up to 6 diopters of cylinder

You may not know what these numbers mean off the top of your head, especially if you are accustomed to discussing your prescription in terms like 20/40. But these are the formal measurements doctors use when crafting glasses or contacts, and they are accurate descriptors of visual acuity.

Doctors Weigh In

While the FDA sets the limits for visual acuity, doctors must perform the procedure. Each time they work on a set of eyes, they put their reputations on the line. They also want their patients to make the best decision possible. So they have opinions about when surgery is an exceptional idea.

An opthamologist is listening to the patient in an exam room.

Overall, LASIK is a very safe surgery, and complications are incredibly rare. But some people face side effects, including:

  • Dry eyes. For most people who experience this issue, it resolves within a few months as the eyes heal.
  • Light sensitivity. Some people experience haloes or starbursts around lights, and others find it difficult to move from a darkened room to a bright one. Again, these issues generally resolve in the weeks and months following surgery.
  • Poor close vision. Some people struggle to see items held close to the face after surgery. Also, LASIK doesn’t protect against normal vision decline associated with aging. People may get LASIK and still need reading glasses as they age.
  • Infections. LASIK is a surgery, and without proper follow-up care, some people develop bacterial colonies in the eyes. If the eyes are cared for properly following surgery, infection is highly unlikely.

Doctors must weigh the risks and benefits in their patients. Some doctors say that moderate nearsightedness, which they define as -5 to -6 diopters, is a sweet spot for LASIK. Patients with vision like this have a potentially big benefit with smaller risks. But people at the limit may also have significant benefits ahead, these doctors say.

People with legal blindness may agree with that assessment. One man wrote a testimonial about his surgery, and his vision measured -8.5 diopters prior to LASIK. While his recovery was long, he now reports almost perfect vision. And he encourages others with vision issues like his to consider a surgical consult.

Vision Testing Before LASIK

You may believe that you know exactly how well you can see. But you can't walk into a doctor's office with a printed prescription and sign up for surgery. You will need a thorough exam, and that should include a comprehensive test of your vision.

A pre-surgery exam can help your doctor plan for your procedure. The results can help your doctor pinpoint what approach to use, what tools would be best, and how your recovery might move forward. But as the American Refractive Surgery Council points out, the consult can also help your doctor determine if surgery is even appropriate for you.

Your doctor may assess your vision using standard tools. But, experts say, some doctors add a special eye drop to the mix. This medication temporarily paralyzes the muscles around your eye, so your doctor has a crisp assessment of how well your eye structures work without the assist of muscles.

refraction testing

The American Academy of Ophthalmology (AAO) says that your doctor may also test your:

  • Contrast sensitivity. This helps your doctor determine how well you can discern colors and shapes.
  • Corneal thickness and shape. LASIK surgeries involve cutting or scraping away tissue from your cornea. Your doctor must ensure enough is left behind to support a healthy eye.
  • Pupil size. Your doctor will take measurements in the light and the dark, to assess your risk of glare and haloes after surgery.
  • Tear production. Your doctor can ensure that your eyes produce enough tears to support healing after surgery.

What Happens Next?

About 800,000 refractive surgeries like LASIK were performed in 2010 alone, says AAO. All of them started with exams, but what happens next is up to you and your doctor.

After a comprehensive LASIK exam, you are aware of just how well you can see and how well your eyes might tolerate a surgery with lasers. Your doctor will go over the results with you in detail, and you will have a chance to ask any questions that come up.

If you choose to move forward with LASIK, your surgery will be scheduled, and your doctor will tell you how to prepare. But if LASIK is not right for you, another type of surgery might work a little better. Or you might choose to stick with glasses or contacts for now. Your doctor can help you understand all the options, so you can make the best choice.



Are You a Candidate for LASIK? 5 Guidelines You Should Know. (April 2018). American Refractive Surgery Council.

To Anyone Considering Laser Eye Surgery. (August 2016). Medium.

Your LASIK Consultation: A Deeper Look Into What to Expect. (July 2017). American Refractive Surgery Council.

Preoperative Evaluation for LASIK Surgery. (January 2020). American Academy of Ophthalmology.

Eye Health Statistics. (2015). American Academy of Ophthalmology.

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