Bifocal glasses have the lenses with two powers meant for near vision and distance vision. The upper segment of the bifocal glass has power which corrects distance vision. The lower segment of the glass corrects near vision.
When we read something, for instance, we look down and see through the lower segment of the glasses. When we look into distance, the line of sight passes through the upper segment.
How are bifocal glasses different from progressive glasses?
Bifocal lenses usually have a visible demarcation line between the two segments. The progressive glasses and single vision glasses (for distance or for near) on the other hand do not have any demarcation line. It’s not visible in progressive glasses because the power changes gradually from the lower part of the lens (for near vision) to upper part of the lens (for distance vision).
When do we need bifocal glasses?
Bifocal glasses are prescribed in two scenarios:
- When someone needs correction for distance as well as near vision and he/she wants to use only one pair of glasses rather than two – one for distance and one for near. As mentioned above, such people would need bifocals with upper segment for distance vision correction and lower segment with near vision correction.
- When someone needs correction only for near vision (the condition is called presbyopia) and does not want to wear and remove reading glasses repeatedly to meet the frequently changing demands of looking at distant objects and near objects in daily activities. Such cases need bifocal lenses with plain (no power) upper segment and near-vision correcting power in the lower segment.
Alternative to bifocal glasses
An alternative to bifocals is progressive glasses. The disadvantage of bifocal glasses is that they correct near vision and distance vision only. The person with bifocal glasses is not able to see things at intermediate distance clearly. For example, with bifocal glasses, you can read a paper or phone in your hand and you can see everything at the other end of the room clearly but you won’t be able to read the text on a table-top computer (intermediate distance).
Since progressive glasses have powers for intermediate distances also, you see everything clearly irrespective of the distance between the object and eyes. For this very reason, progressive glasses are generally preferred to bifocal glasses.
Compared to bifocal glasses, progressive glasses can cause eye strain and headache if not used properly. Read this post to understand how.
Photo by Muffet