[Jack]

Happens to us all, eventually.
Fabs, on the glasses, maybe, but, when you get to the bifocal stage, it gets difficult. Often, if your head position is correct for getting your cheek on the stock, it is incorrect for your bifocals.
And, with or without glasses, the ability of the eye to accomodate lessens with age. Accomodation is the ability of the eye to rapidly change focus from the rear sight to the front sight to the target, and back again. When you're young, your eyes can do that so fast, your brain is fooled into thinking that all the sights are in focus at the same time.
With age, you lose that ability.