Can you explain the difference between no-line bifocals, progressive bifocals and trifocals? I was told at a discount eyewear store that there was no difference and that all three of these glasses were essentially trifocals. Generally, we think of vision in three distinct distance ranges: near for reading, intermediate for computer, card playing, etc.
A bifocal provides clear vision in any two of these ranges of vision, whether it has a line or not. A trifocal has lines and provides vision in all three ranges, but there is often blurring of vision between these three focal distances.
Think of each focal range as a step in a stairs. A progressive lens provides good vision in all three ranges like a trifocal, but has no lines and provides clear vision in the spaces between the three steps of a trifocal. A progressive lens is more like a ramp than it is individual steps. I suggest you ask your ophthalmologist to go over this with you. This question was originally answered on Jan.
Bifocals and trifocals are both eyeglasses. They come with the added benefit that the same glasses can be used to view objects at different distances. They differ in the number of optical power regions they have. Bifocals, as the name suggests, have two regions of optical power. Trifocals, on the other hand, have three different regions of optical power.
Bifocals have a lens that is divided into two parts. Trifocals contain a lens that has an additional region apart from the two that are already present in bifocals. For example, this region comes in handy when working on a computer.
Bifocals were invented in the 18th century by Benjamin Franklin. Trifocals on the other hand, were developed much later in the s. Initially bifocals were made simply by joining two different lenses together. Because of this the early glasses developed in such a manner were quite fragile.
No such thing was done for the trifocals. Trifocals and bifocals are usually prescribed to people with presbyopia. The two sections of the lens are usually separated by a visible line.
The segment of the lens devoted to correcting near-vision can be in several shapes including a half-moon, a round segment, a narrow rectangular area, or the full bottom half of the bifocal lens. With only two zones of vision, bifocal lenses do not account for the intermediate zone of vision. Since this intermediate viewing area is about 18 to 24 inches from the face, it can be difficult to view things like a computer screen while wearing bifocal lenses. In an effort to correct the problem of lacking an intermediate viewing area, trifocal lenses were invented.
Trifocal lenses offer correction for near and far vision, but also enable a person to see clearly at an intermediate level about the length of your arm. Trifocals help correct the intermediate zone by featuring a second small lens segment directly above the area used to correct near vision. Presbyopia is usually caused by a stiffening of the eye tissue that prevents the eyes from focusing on objects close up.
If you are currently wearing prescription glasses, being diagnosed with presbyopia generally means that you will need to start wearing multifocal lenses so you can continue to see clearly at all distances. Bifocal, trifocal and progressive glasses are all designed to improve vision for people with presbyopia.
All three of these solve the problem of needing a different lens curvature for correcting distance viewing than they would for close-up reading.
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