This is actually something I had been meaning to ask about… my understanding of far-sightedness was that the eyeball is too short (for the focusing system), and so the near point is pushed out. But as long as it’s not pushed out as far as 20 feet, wouldn’t they be able to see fine at 20ft ? They would have to accommodate more than a “regular” person to do so, but that should still be okay, shouldn’t it ? (I’m using the term accommodate to mean activate ciliary muscle to increase focusing power, rather than possibly the more general term of just adjusting the focus.)
I’ve not thought much about far-sightedness, so have probably got all this wrong… this is just me thinking out loud, and possibly making a fool of myself.
Is the purpose of differentials to be able to view (eg) screen in a relaxed state. In my odd view of the world, glasses move the real world forwards or backwards to map it into the range your eye can focus at. For a myope, differentials map screen-distance to blur-distance, whereas full prescription would map it much closer (since it has to map infinity to the blur point, and therefore fit all of the rest of the world into the limited space between there and your eye).
For a hyperope, the full prescription “pushes the world away” a little, so that things that are close in the real world are mapped to your near-point blur horizon. But that doesn’t push your computer screen very far away. in order to put the screen at the fully-relaxed point, you’d want to push it away much further than usual (to infinity and beyond, as it were…) So yes, I guess that would require a stronger than usual correction. Things closer than your near point would become visible. I guess things beyond the screen would become blurred. (Err… myopic blur, I guess, since they’d be focused in front of the retina …?)
In order to use differentials to actually exercise your AF when your far-sighted… that might be a bad idea. Dunno. Does Jake say anything about farsightedness in the endmyopia program ?
The problem is that to induce an eye-lengthening stimulus, you’d presumably need to look at something such that it is focused slightly behind the retina. But the eye’s going to be fighting you by activating the ciliary to try to resolve the blur. So you’re going to have to be at full accommodation in order to provide this stimulus. (Have I got that the right way round…?) If you set up your glasses and your screen to try to provide this stimulus for hours at a time, you’re presumably going to get ciliary lock-up. Which I think would be a bad thing.
Myopia, has the convenient property that AF happens at the point where your eyes are relaxed anyway. (For a suitable definition of relaxed.) So if you’re wearing your differentials and lean back from your screen slightly, you get a blur and you can practise your AF. (Note, this isn’t from personal experience - I’ve not yet got to grips with AF.)
Does an analogue of the endmyopia program even work for far-sightedness ? I’m trying to think what would be the equivalent of lens-induced progression. What could you be doing to cause myopic defocus that would stimulate the further shortening of the eye ? ISTR far-sightedness is usually just because your eye didn’t lengthen enough during early development.