Starting around the age of 40, we loose the ability to see objects at very close distances. We might first notice this when we have difficulty in reading the small print on our medicine bottles or legal documents. As we grow older near vision will slowly become progressively more blurry, but distance vision will not be affected. This inability to see near objects is called presbyopia.
We have two types of spectacles to help people with presbyopia see near objects. These are Bifocal and Progressive lens. The major physical difference between the bifocal lens and the progressives lens is the demarcation line between the area used to see far and area used to see near objects. The main functional difference is that bifocal has only two focal points, while progressive has many.
Bifocal as the name implies have two areas of focus- the top portion for distance and the bottom circle for near. The reading power can be titrated to the distance that the person requires. For a person who read books in his hand would need the focus to be at 20-30 cm, while if he reads with the paper on a desk while he leans back the focal distance would be 3-40cm. If he requires to see both near and intermediate objects/ print then he has to opt for a progressive lens.
Progressive glasses are also known as varifocal, graduated glasses and no-lines bifocals. These lenses have more than one focal point or lens power with different powers at different levels of the lens. At the top of the lens, the power is minimum, which gradually increases in the middle level and finally, the bottom part of the lens has the maximum power.
We have two types of spectacles to help people with presbyopia see near objects. These are Bifocal and Progressive lens. The major physical difference between the bifocal lens and the progressives lens is the demarcation line between the area used to see far and area used to see near objects. The main functional difference is that bifocal has only two focal points, while progressive has many.
Bifocal as the name implies have two areas of focus- the top portion for distance and the bottom circle for near. The reading power can be titrated to the distance that the person requires. For a person who read books in his hand would need the focus to be at 20-30 cm, while if he reads with the paper on a desk while he leans back the focal distance would be 3-40cm. If he requires to see both near and intermediate objects/ print then he has to opt for a progressive lens.
The reading segment can be round, D shaped or a flat line as in executive bifocals. Round shape is most comfortable while D shape gives more area for distance. Executive bifocals have large area for near for people who read a lot.
Progressive glasses are also known as varifocal, graduated glasses and no-lines bifocals. These lenses have more than one focal point or lens power with different powers at different levels of the lens. At the top of the lens, the power is minimum, which gradually increases in the middle level and finally, the bottom part of the lens has the maximum power.
Commercially, progressive glasses are quite popular because of the absence of any line on the lens. The greatest advantage of progressive glasses is the ability to look at objects at varying distances with just a slight tilt of head. As the different levels have different powers, the top part can be used to view objects at a distance, the middle one at the objects at an intermediate distance and the bottom part while working on the nearby objects, like reading a book. They also avoid the discontinuities like image-jumps which occur in the visual fields created by bifocal and trifocal glasses.
