From Grant Cunningham in the Gun Digest Book of the Revolver, commenting about his deteriorating eye sight:
"I discovered that if I focused on the target, instead of trying to focus on my front sight blade, I could shoot surprisingly accurately. Heresy! The reason is simple: alignment guides still align things even if they’re not in perfect focus. The sharper the focus, the more precise the alignment, that’s all!
Having the front sight in focus gives us the most precise sight alignment, but having the target in focus gives the most precise sight picture. A loss of precision in one seems to be compensated by a rise in precision in the other. The technique is simple: focus on the target. As you do so, bring the sights into your line of sight. Keep your focus on the target, and look through the sights. Now just get the sight alignment/sight picture that you already know. The sights will be fuzzy, but you’ll easily be able to discern when everything is as it should be. That’s all there is to it!"