If you are between 18 and 39 years old, your lens still has its ability to focus. The quality of your vision depends on the shape of your eye. If the curvature of your eye is imperfect and/or if your eye is too long/too short, this results in a defocusing of the image which must be compensated by an optical correction. The ULTRALASIK surgery corrects the imperfections in the shape of your eye with micrometer accuracy by removing thin layers of cells on the surface of the cornea. The ULTRALASIK surgery is intended for young people over 18, who have a lens in the prime of life and for whom eyesight has been stable for at least a year.
If your eyewear prescription, for either glasses or contact lenses, has changed by more than 0.75 dioptre during the last year, you should wait for your eyes to stabilize before proceeding with surgery. Laser surgery does not put a stop to the natural evolution of your vision.