What do eyeglass prescription numbers mean?
Eyeglass prescriptions generally have 3 numbers on them that include:
1) Spherical power
2) Astigmatism Magnitude
3) Astigmatism Axis
Spherical power and astigmatism magnitude are both values given in diopters. Positive spherical values generally mean you have far-sightedness, or hyperopia, meaning you see better far away than you do up close. Negative numbers typically mean you have near-sightedness, or myopia, and that you don’t see well far away but have good near vision. Near-sightedness is much more common in the USA and about 80% of LASIK procedures are performed for myopia.
Astigmatism means that your eye does not focus images properly on the retina, most commonly because your cornea (clear front structure of the eye) is steeper in one direction than the other. You will often hear this described that your eye is shaped more like an American football with 2 differentt curves and less like a baseball that is overall shaped like a sphere.
Most people have some level of astigmatism but it is often under 1 dioptre and when the value is 0.50 dioptres or less than it is often not corrected in your glasses or contact lenses. In this situation your glasses prescription will usually have 0.00 or sph written in the astigmatism portion of the grid. Sph is short for “spherical” meaning the shape of the eye is that of a sphere.
The third number is axis of astigmatism. Since people with astigmatism have 2 different curvatures either the steep angle of curvature or the flat angle of curvature must be listed on the prescription so that your glasses or contact lenses can correct this variation in corneal shape. If there is no astigmatism then this box on the grid will be left blank.
The above video helps explain the concept of astigmatism.
Can everyone achieve better vision with vision correction surgery no matter what their eyeglass prescription numbers are?
Most people, regardless of nearsightedness, farsightedness or astigmatism achieve better vision with LASIK or other forms of vision correction surgery like SMILE, PRK, RLE, or ICL. We are very fortunate to live at a time with a wide variety of surgical procedures that are very effective at treating a wide variety of refractive errors.
More FAQ’s about Your Eyeglass Prescription Numbers: What They Mean and What Vision Correction Procedure Can Help
What does your prescription have to be for LASIK?
Myopia, or nearsightedness, is generally correctable with LASIK up to about 8-9 diopters of correction. Most modern LASIK lasers are actually FDA approved up to 12D of myopia correction but you will find most LASIK surgeons won’t go that high because these treatments often require excessive cornea tissue removal and may lead to low quality of optics past about 9 diopters. When your corrective lens prescription is beyond 10 diopters most US surgeons will recommend treatment with the new Evo ICL from STAAR surgical.
Hyperopia, or farsightedness, is generally correctable with LASIK up to about 4 diopters. Again, the FDA has approved even higher corrections. The Alcon Wavelight EX500 with Contoura technology at Las Vegas Eye Institute is actually FDA approved up to 6 diopters of farsightedness but Dr. Swanic is unlikely to recommend going beyond 4 Diopters of correction. This is because most studies show that corrections are not stable and lead to regression when you go over 4 diopters. The Alcon Wavelight platform is largely considered to be a much better and more stable platform than the Visx platform for treatment of hyperopia. The Visx is also approved for 6 diopters in its “conventional mode” and its latest iDesign treatment profile is approved to 4 diopters. Las Vegas Eye Institute switched from Visx to Wavelight last year partly due to the Wavelight’s better treatment of hyperopia. We strongly recommend that if you are considering hyperopic LASIK you consider options other than LASIK past 4 diopters. In the US the main treatment option is RLE, or refractive lens exchange, for hyperopia above 4 diopters. Fortunately, most people seeking LASIK correction for hyperopia are under 4 diopters and will do well with the Wavelight or iDesign platform.
As a sidenote, LASIK for hyperopia leads to better optics if it is centered on the visual axis instead of the pupil center. The reason for this is that the vast majority of hyperopes have the center of their vision nasal to the center of the pupil. To enable this on an Alcon Wavelight laser your surgeon needs to have the newest EX500 laser and the topoylyzer capture device. You need to be captured with the topolyzer device for your surgeon to enable this feature. Las Vegas Eye Institute has both the EX500 and topolyzer device and shifts all of its treatments toward the visual axis rather than the pupil center.
The above video explains correction of hyperopia or far-sightedness.
Can you get LASIK with high prescription?
Yes. LASIK as explained above actually can correct over 80% of the people who visit Las Vegas Eye Institute for near-sightedness and farsightedness. LASIK is a agreat option for nearsightedness up to 8 diopters, farsightedness up to 3-4 diopters, and astigmatism up to 5 diopters.
What is the maximum prescription for PRK?
PRK stands for photorefractive keratectomy. It is performed with the same LASER that is used for LASIK. The only difference is that with LASIK a flap is created, the treatment is performed under the LASIK flap, and then the LASIK flap is returned to its original position. Some surgeons refer to PRK as ASA which stands for Advanced Surface Ablation which generally just implies that the procedure is performed with an advanced LASIK laser that performs a wavefront guided, wavefront optimized, or corneal topography guided approach.
PRK is generally performed at lower prescription levels than LASIK. The reason is that PRK has a higher incidence of corneal haze formation at higher corrections. At Las Vegas Eye Institute we generally limit PRK treatments to 7 diopters or less for myopia and 3 diopters of hyperopia. We perform all of our PRK procedures with MMC (mitomycin-C) to decrease the risk of haze formation. PRK haze is associated with UV light formation. Given the high amount of UV light in the Las Vegas valley we recommend MMC on all treatments performed here. When Dr. Swanic trained at Tufts, in Boston, some doctors recommended only using MMC on treatments of over 2.5 diopters, but Boston gets much less sunlight than Las Vegas!
Is laser eye surgery based on prescription?
Yes. LASIK procedures are programmed in almost an identical format to a glasses prescription with input of spherical and astigmatism values.
What prescription can LASIK not fix?
LASIK cannot fix eyes with high levels of “irregular astigmatism.” Irregular astigmatism is astigmatism that is not uniform in shape. Astigmatism is a measure with a device called a corenal topographer. Regular astigmatism tends to look like a “bow-tie” on corneal topography but irregular astigmatism looks like a bowtie that isn’t tied evenly. Alcon’s Contoura technology, used at LVEI, is the most advanced version of topography guided LASIK correction available in the USA. The only other FDA approved LASER for topography guidance is the NIDEK EC-5000 which was originally FDA approved in 1998!
Irregular astigmatism can be caused by disease of the eye or cornea and in those cases LASIK should not be performed. Keratoconus is a weakening of the cornea and should not be treated with LASIK. Anterior basement membrane dystrophy (often abbreviated ABMD) is another relatively common condition that can cause irregular astigmatism and is occasionally detected on LASIK screenings. Fortunately, this condition can be treated prior to LASIK at LVEI. If the condition is very minor then PRK can be performed and treat the refractive error and ABMD at the same time.
The above video discusses topography guided LASIK which can correct low levels of “irregular astigmatism” as long as it is not due to keratoconus.
What can disqualify you from getting LASIK?
People with moderate to severe dry eye should not get LASIK as it will often worsen this condition. Your LASIK doctor should screen you for dryness during your preoperative eye examination and you should be sure to tell your doctor if you have significant issues with dry eye. Some patients with eye dryness can be treated prior to LASIK and still have a very successful outcome. Eye dryness is often treated with punctal plugs, anti-inflammatory drops (Restasis,Cequa,Xiidra, etc), and oral omega-3 supplements or oral antibiotics like doxycycline.
It should be noted that some people with significant dryness sensation with contacts will often do quite well with LASIK or PRK when they are no longer wearing contact lenses. Dr. Swanic actually had laser vision correction because he could not tolerate wearing his contact lenses for more than 2 hours and he has done great post laser vision correction. Contact lens intolerance is actually one of the leading reasons that people seek LASIK consultations in the USA.
People with kertoconus as discussed above should not undergo LASIK as it can severely worsen with LASIK. Las Vegas Eye Institute uses the most advanced diagnostic testing devices in the world to screen for keratoconus. Specifically, all of our patients undergo Cassini Topography, Galilei G4 Tomography, iDesign wavefront testing, and Zeiss Cirrus 6000 epithelial thickness mapping during their LASIK consultation.
How long does it take to get 20/20 vision after PRK?
PRK typically takes 1-2 months to get to 20/20 vision. The amount of time varies based on level of myopia correction. In myopia over 5 Diopters most patients require 6-8 weeks to reach 20/20, while in myopia under 3 diopters most patients acieve 20/20 vision in 4 weeks. Patients between 3-5 diopters are often more variable and typically require somewhere between 4-8 weeks.
Can 1.75D eyesight be cured?
Yes. 1.75D of myopia, hyperopia, or astigmatism can be treated with modern LASIK and PRK procedures.
What myopia is too high for LASIK?
Beyond 9 diopters of myopia most LASIK surgeons in 2023 will instead recommend the Staar Evo ICL.
Can you get better than 20/20 vision with PRK?
Yes. You can get better than 20/20 vision with both LASIK and PRK. Most patients (over 90%) with modern wavefront optimized, wavefront guided, or topography guided LASIK will achieve 20/20 or better vision.
What is the best surgery to improve vision?
The answer to this question essentially depends on your current situation. People with typical prescriptions between 6 diopters of myopia and 3 diopters of myopia will do best with PRK or LASIK with the lowest risk of complications. The ICL procedure will be the best procedure for patients with over 10 diopters of myopia. Refractive lens exchange, or RLE, will be the best procedure for older patients that don’t yet have a cataract but have a lens that no longer functions well to see up close (typically this is patients over the age of 55-60.). RLE is often the procedure of choice for people with hyperopia above 4 diopters. However, all people are unique and specific eye characteristics may make any of these procedure more or less appropriate for any one patient.