For LASIK, it depends upon mainly corneal thickness.  Some patients have thin corneas, and that could make them poor candidates.  Extremely nearsighted, far-sighted, or highly astigmatic candidates may not be candidates (their better option would be lens replacement).  Patients with a history of dry eye are poor candidates for LASIK, as the procedure may worsen symptoms.  Diabetics are also considered poor candidates due to a more unpredictable corneal response.  For lens replacement, corneal thickness is a non-issue, and the procedure can correct higher refractive errors than LASIK.  If there is astigmatism, often the surgeon will perform a LASIK procedure just for the astigmatism and the rest will be corrected with the implant.  Patients over 40 who have LASIK will still need glasses to read, unless they have it set up in monovision (one eye distance, one eye near).  Patients who have lens replacement using an accomodative implant like ReStor should rarely if ever need readers.