Bifocal eye glasses work by accommodating eyes that are both nearsighted, or myopic, and farsighted, or hyperopic. This means a person requiring both prescriptions is presbyopic.
What bifocal lenses do is bring the eyes to near perfect, if not perfect vision. Perfect vision is equated to 20/20 vision, meaning a person can see objects clearly at 20 feet away.
Note, unlike bifocal vision, single vision means a person needs one prescription in their lenses only. The lenses can have matching prescriptions or different ones. These can consist of a plus or minus prescription, or even no prescription in one eye, while the other has either a plus or a minus prescription, or both.
Bifocal vision means a person needs two separate prescriptions, one to see clearly at a distance and a different one to see up close. The person is both nearsighted and farsighted.
Trifocal vision means a person needs three separate prescriptions, one to see clearly at a distance, a second one to see intermediately at about an arm’s distance and a third one to see up close.
Myopia is an eye condition where a person sees clearly up close but has trouble seeing objects at a distance. This is also known as nearsightedness. The eye’s cornea has steep curvatures causing it to bulge outward. This is also known as the eye having “too much plus.” As a result, light rays entering through the cornea don’t come to rest at one focal point on the retina. Instead, the light rays come to stop before reaching the retina causing blurred vision. A minus prescription is needed to subtract or cancel out the additional plus and push the light rays’ back to the retina coming to rest at one focal point. This results in the eye seeing clearly.
Hyperopia is an eye condition where a person sees clearly at a distance but has trouble seeing things, especially print, up close. This is also known as farsightedness. The eye doesn’t have enough plus in the eye, but too much minus causing light rays to focus behind or past the retina. A plus prescription is required to pull the light rays back, so to speak, cancelling out the excessive minus in the eye. This causes the light rays to focus on the retina at one focal point so the hyperopic eye can see clearly.
Presbyopia means the eye requires a plus and a minus prescription to see clearly at a distance and up close. Someone with presbyopia is both myopic and hyperopic. Bifocal eye glass lenses accommodate a presbyopic person by correcting their distance vision and near vision with one pair of eye glasses.
Types of Bifocal Lenses
A traditional bifocal lens has two different prescriptions incorporated into it. One is for distance vision and the other is for reading or near distance. The distance prescription is spread throughout the entire lens with an ADD power placed in a segment located at the bottom of the lens to accommodate the near vision, or the reading portion.
An executive multifocal lens (bifocal or trifocal) is one in which the segment(s) extend all the way across the lens. This means the reading, or close up range is wider, but it’s also very noticeable, not cosmetically pleasing, and can cause the eye glasses to be quite heavy on the face.
The double segment bifocal lens has a near, or reading portion on the bottom and the top of the lens. Double segment bifocals may be used by people in construction, for instance, who need to see close up while looking down or up.
A blended segment lens is a bifocal lens without a line, mostly for cosmetic reasons.
The progressive addition multifocal lens is known as a bifocal lens without a line or no-line trifocal lens. Not everyone can wear progressive lenses, and it can take a couple of weeks to adjust to them for some wearers because the lenses have three sections. The power gradually increases from the distance to the near vision in the central corridor, which is where the intermediate prescription is located.
Progressive lenses are an all distance lens, and are also known as progressive addition lenses, lineless bifocals, no-line bifocals, and seamless bifocals.