The abrupt
demarcation between the upper--distance--and lower--near--focals in
bifocals,
glasses those who wore ordinary single focal glasses before forty, wear after.
So-called, because it is exactly where sheet music, placed on an organ's music stand rests, precisely in the mid-range; what the performer sees, is foggy in either the near or far focals. It is also a curse for those who play piano. Generally considered an advertisement of old age.
In the modern age, information technology is unable to escape the weaknesses of the flesh; a computer monitor sits in exactly in the same, #$%^*, place.
When I turned forty, I joined this venerable club, though I would deny my own venerability. The solution is simply to grind another, third, focal smack in the middle of the other two, focussed in the mid-range.
Not wanting to advertise this dramatic change in my vision, I have chosen to avail myself of computer-assisted grinding machines which permit the smooth, line-less transition between all three focals.
Not totally smooth in its production, the glasses I am wearing are the second set of lenses ground--it took almost a week of trying for them to be made, with delays getting the proper blanks, or whatever they're called, as well as the false start.
And I had to learn not to follow along with just my eyes, because they would track across the much more complex focality of my new lenses, bringing fog and confusion.
Now I turn my head almost by reflex--I can no longer be subtle when following something interesting. At my age, however, I may no longer have time for subtlety.