Around the age of 40 years, when you start having difficulty in reading small print and seeing near objects well (the condition is called Presbyopia), corrective glasses are needed to see close objects clearly. The glasses may be used only for correction of near vision in which case the glasses have only the power of the lens for near vision. In another scenario, the glasses may also include the power to correct distance vision (in the upper segment of the glass) and a different power to correct near vision (in the lower segment of the glass).
The glasses which have lenses meant only for near vision correction, popularly known as ‘reading glasses‘, have a fixed power (for example, +1.25 Diopter for both eyes). Reading glasses help you see things clearly at a convenient up close distance like a book, a phone screen and laptop screen.
The glasses which include lenses for both distance and near vision may be of two types:
1. Bifocal lenses – These glasses have a lower segment for the near vision (for example, +1.25 Diopter) and the upper segment for distance vision (for example, -0.50 Diopter). There is a demarcation line visible between these two areas representing the change of power between the two segments. So practically, only two zones of the lens are available to make use of.
Normally, the person wearing bifocal glasses would see a distant object through the upper segment and a near object through the lower segment because the near object is usually below the level of eyes. It is important to note here that tilting the head up or down will make you see through the lower and upper segments of the glasses respectively. The users of such glasses subconsciously tilt their head up and down to choose between clarity of near objects and distant objects.
Since the difference between the two segments is ‘noticeable’, the person is always conscious and aware of the segment (upper or lower segment of the glasses) to see clearly. This is an advantage of bifocal lenses – in the later part of this post, you will understand why.
The disadvantage of bifocal lenses is that the near vision is corrected only for a narrow range. Anything farther and closer than this narrow range is blurred. Progressive lenses were developed to solve this problem.
2. Progressive lenses – These lenses do not have two distinct zones like bifocal lenses. Instead, the lower zone meant for near vision correction gradually blends with the upper zone for distance vision correction (first image above).
For example, lower zone with +1.25 Diopter ‘progressively’ changes to -0.50 Diopter in the upper zone with infinite number of powers between them. These available powers of the lens help in seeing objects even at ‘intermediate’ distance.
So if a person with bifocal lenses was able to see near objects clearly only within a narrow range, he/she with progressive glasses will see things clearly also at intermediate distances. As an example, progressive glasses will help you see everything clearly including the piece of paper with fine print in your hand (near distance), the computer screen at your desk (intermediate distance) and objects at the other end of the room and those outside the window (far distance).
It’s important to remember that to see anything clearly at any distance; the progressive glasses user has to tilt his/her head up or down to see through the appropriate zone of the lens. Therefore if the person has to see the computer screen after reading something on a piece of paper in the hand, he/she will have to tilt the head down a bit to see through a zone of lens which has less power (for example, +0.75 Diopter) than that for near vision (which in our example is +1.25 Diopter).
Now let’s talk about the disadvantage of progressive glasses. Since there is no demarcation line between near and distance segments in the lens, we usually are not aware of our head tilt and therefore which segment of the lens we are using to see anything.
For example, while working on a computer especially for long hours, you may be looking at the screen through the upper zone of the lens (actually meant for distance vision). You may not notice this and it will lead to eye strain and headache because the intrinsic muscles of the eyes go into spasm in an effort to compensate for the poor vision. That is the reason why some people complain of eye strain in spite of using correctly prescribed glasses.
Therefore the most important point to remember about using progressive glasses is to ensure looking through an appropriate zone/segment of the lens by maintaining a corresponding head position/tilt. If you are new to progressive glasses, keep trying until you develop reflexes to tilt your head to see everything clearly. Getting used to these glasses does take some time.