You must be myopic, and your optimal distance of your optical system is naturally at near. If so you may not need bifocals at all. Think of your distance glasse

You must be myopic, and your optimal distance of your optical system is naturally at near. If so you may not need bifocals at all. Think of your distance glasses power as making up for how much your optical system is off by to be focused at infinity (distance). A person with a minus prescription has excess power and naturally focuses nearer targets. A person with -2.00 power glasses is focused best at 1/2.00 = 0.5m which is a nice reading distance. A person not requiring glasses ("0") is focused at infinity and has perfect distance vision. For such an eye to focus up close the lens/ eye muscles work to increase the power (fatten the lens) until near is in focus. Unfortunately this ability to accommodate steadily decreases w/ age, noticeable at 40yrs old. Those who are +3, the focal length is 1/3meters BEHIND your eyeball and you'll need glasses for everything once your natural ability to vary focus starts dropping off. You can figure out your exact focal length (meters) by taking your distance prescription a + b x c, and calc 1/(a + 1/2b). minus in front of eye, plus behind the eyeball. Neat, huh?