In the PRK-MMC group , the mean preoperative spherical equivalent refraction was -3.84 1.59 diopters ( D ) ( range -1.5 to -8.7 D ) and -0.21 0.44 D ( range -0.5 to 1.0 D ) at the last follow-up and in the epi-LASIK group , -3.91 1.48 D ( range -1.5 to -8.5 D ) and -0.18 0.36 ( range +0.5 to -0.5 D ) , respectively .