Most patients who undergo refractive surgery to correct imperfections in their vision come out of the procedure with eyesight that is 20/20 or better. However, researchers have come to realize that fixing imperfections can affect vision in unexpected ways — some patients exit the surgery slightly farsighted or nearsighted. A study conducted five years ago on 340 eyes showed that 74 of the eyes treated, or 21.8 percent, were slightly farsighted. At the European Society of Cataract and Refractive Surgery meeting in London in September, Dr. Scott MacRae of the University of Rochester Eye Institute presented the results of a new study that shows a significant drop in farsightedness among patients who underwent lasik surgery. MacRae and his colleague Manoj Venkiteshwar developed a formula called the University of Rochester Nomogram that predicts which patients are more likely to be farsighted after the lasik procedure and that adjusts the laser accordingly. The complex computer formula controls how the laser beam, which hits the cornea at about 50 times per second with 750 to 3000 pulses, moves around on the surface of the cornea. The surgeon can then sculpt the cornea into the right shape so that it produces as flawless an image as possible. MacRae and his colleagues at the university tested 445 eyes using the formula and found that only six eyes, or 1.3 percent, were slightly farsighted after lasik — a dramatic decrease from the results of five years ago. The researchers plan to continue working to improve patients’ vision, building on the work of David Williams, also at the University of Rochester. Williams first developed the system that enables doctors to see subtle imperfections in the eye.