Hi Peter,
You may wish to try out a set of "ghost rings" for one of your rifles. Jeff Cooper writes, "Several old timers now have reported that this sighting system does wonders to make up for the increasing stiffness of the cornea that normally comes with age." You may be interested in trying this out on one of your rifles and finding out if it works well for you. (Be sure you don't confuse "true" ghost rings with mere aperature sights. Some gun writers may not grasp that not all aperature sights are ghost rings.) FWIW.
Stay safe,
J.T.