Astigmatism, glasses, contacts, the whole buffet. My age will start with a "3" later this year. FWIW, I have the basic Defoor sights on all my Glocks and have found no detriment to what I can do. The proctor sights are a close second but I find them noticeably slower. I find Trij HDs and the like way too busy. While I don't have the experience level of some when I was training with Defoor he mentioned several of his old work buddies with eye issues liked either all black sights or were moving to the RDS.