What's the difference between the parts of a bifocal lens?

Question

In other words, why would someone need different focal correction for near vs far or reading vs driving? Is it correcting two unrelated focal defects? Or do such defects have a characteristic that the same defect needs different correction for near vs far?

Answer 1

A bifocal lens is actually two different prescriptions cut into the one lens. The bifocal portion is actual a magnification lens, and the upper portion can either correct for near or far vision, as needed. They are a form of progressive lens, and originally bifocals required two different convex lens to be cut and then stuck together. Nowdays, of course, they don't have to do that. The magnification usually starts at a line just even with the bottom eyelid, and progressively provides stronger magnification as the lens goes downwards. Presbyopia really only affects vision within 3 feet or so of the eyes, when the pupil will naturally drop so you can read/focus on something you have in your hands. That vision may still need correction for objects at a distance, or even nearer- which you would be looking straight ahead or sideways to focus on. So the upper lens can be ground to provide that correction, in the direction the eyes will be directed. In some cases, people can have normal vision except for the presbyopia, and can have a bifocal with no additional correction in the upper lens at all. The reading glasses sold in stores, on the other hand, are only ground for the magnification part of the bifocal. The magnification is measured in standard units, usually in gradiations of .25, and you can purchase straight bifocal/reading glasses usually as low as .75, for folks who really only need them for reading. Or folks who don't want the giveaway look of a bifocal lens. There are also trifocals, another variation of the progressive lens for folks with presbyopia, but who are in need of correction for both near and far vision. Those are a bit trickier to adjust and adapt to, because you don't have the correction you need for all three problems in any one section of the lens. As a rule, the upper portion of the lens will correct far vision, the middle section will correct for near vision, and of course the bifocal will be an insert at the bottom. In short, you are talking about correcting three different visual defects- and having one doesn't automatically infer having one, or both of the others. I've worn bifocals since I was a teen, but didn't require correction for my near vision until some years ago. I worn reading glasses for many years, and only needed them for reading, needlework, or eventually when I became a nurse- to focus on the syringes and such. Until I became a nurse, I didn't wear them full time- didn't need to. Even then, I only wore them at work full time. Otherwise, I wore them as I needed.

Answer 2

The eye's ability to refocus for different distances is maximal at about the age of eight, and drops off almost linearly from there.
Somewhere around 45, (but with a large individual variation, and ethnic differences) this refocussing ability is insufficient for prolonged comfortable closework, and a +ve lens is needed to allow closer focus.
This applies whether or not there is a distance correction, and in a bifocal, this +ve "addition" is added to whatever distance power is needed to produce the same effect as a normal- sighted person using a pair of simple reading glasses.

Very rarely the near Rx isn't a simple focus-modified distance Rx, but needs individual prescribing. There, either bifocals are avoided, or the ancient device of the "Franklin split bifocal" is employed. Essentially four half lenses mounted together in one frame!

Answer 3

Point of reference is key here- Mathematically speaking, your bifocal Rx is dependant upon what the distance correction is whether it be near or farsighted- and then the bifocal power is added to that.


Hope this helps!

Answer 4

DT you're sure and desperate to be responsive to my age ..LOL yet specific been donning glasses for the reason that i grow to be 10( oh me what an age to commence something) And now I could desire to have bi's too. My eyes are fairly gentle to the sunlight and positioned on some clip-ons. Cant positioned on contacts, as I actual have tried them 2 situations. putting them in wasnt the prob. or taking them out.. I actual have dry eyes. so because it rather is me and my eye issues. a minimum of i can be certain and not blind which i'm greatful.Oh and that i've got staggering brown eyes..