Some people who wear glasses have tried those cheap reading-aid glasses, and they say well yes they are indeed magnifying glasses but they never work quite well for me regardless of value I choose. Whether +1.00, +1.50, +3.00 whatever, they are not quite right.
Well, of course they won't be right for you on their own in general, but try the following easy hack:
If you combine your normal prescription glasses (so those you got prescribed and would wear to see far away/infinity in sharp focus,) and you overlap any of those cheap +2.50 "reading-aid" glasses on top of those, *only then* you will get a perfectly comfortable "reading distance" combination, that is, when the reading material is placed at about a forearm's length or the so called Harmon distance (~40 cm.)
That works because typically the ideal "+ADD" eye-prescription values for Reading (books or a smartphone, let's say) are between +2.00 or +2.50. They call this value "ADD" for a reason: it has to be ***ADDed*** to your base prescription.
The cheap reading-aid glasses work well on their own only for people who don't need any glasses to see infinity in sharp focus. Yet this hack overlapping those glasses onto your own glasses works pretty much perfectly, regardless of your prescription.
If instead of +2.50 you use a +1.50 pair of reading-aid glasses, again overlapped with your normal prescription glasses, then you get the perfect optics not to read a book or your smartphone, but to use a computer, or in other words, to have the sharpest focus at about a full arm's length (or 2/3 of a meter.)
If you want some math explanation, the maximum possible focusing distance in meters (with eyes fully relaxed) will be exactly 1 divided by that ADD value:
1 / 2.50 = 0.40 (== 40 cm --> adequate book or smartphone reading distance)
1 / 2.00 = 0.50 (== 50 cm --> still adequate or even preferable for book or smartphone reading distance for some)
1 / 1.50 = 0.67 (== 67 cm --> farthest possible focus at arm's length --> computer use)
Once again, this works independently of your prescription actually. But that is, assuming your prescription was properly measured.
Because if when you overlap the glasses the way explained above, your sharpest focus turns out to be at a significantly different distance from what was just described, or especially, if the sharpest focus through those overlapped combos requires in fact ***different viewing distances*** for each eye (so one eye let's say perfect focus at 50cm, the other eye however achieves best focus only a few centimeters further away, or a bit closer, but not exactly at that same distance,) then your prescription's SPH values are in fact incorrect and should be revised.
Doing this overlapping hack with reading-aid glasses of +3.00 or more would be useful only if you have to do some really very close-range work, like a watchmaker (maybe some miniature repairs, or very small drawing/carving.) In any case, anything above +2.50 will have quite too high a magnification for normal reading purposes. The true most useful values in general will therefore be just +2.50 for reading, and +1.50 for computer usage. Try this hack for those use cases themselves, and/or to also validate the prescription on your glasses.
PS. I validated this hack with an old friend of mine who is a professional oftalmologist. Her own words: "It’s not actually a hack; that is the correct way to measure the bifocal add." She had however an observation with respect to age. The recommended Add value can be exactly as mentioned above the older the person is, but could be lower taking age into account. The eyes still have focusing compensating power at younger ages. As rule of thumb, for 44 years, an Add value of +1.25 typically works well already, and then an additional +0.25 is added for every four years of age, up to a maximum total Add of +2.50 (for reading.)
