Bifocal glasses are used to correct eyesight where the patient requires more than one focus to correct the vision. This is usually due to requiring distance glasses in addition to presbyopia. Presbyopia occurs when the crystalline lens inside the eye stiffens with age and cannot cange shape to focus close objects.

So the bifocal wearer will need a distance correction of either Myopia (short sightedness), Hypermetropia (long sightedness), astigmatism (rugby ball shaped eyes) or a combination of any of the three. The segmented part of the bifocal will have the same distance correction but with an extra “reading add” for the presbyopia.

There are four shapes available:

D Seg:

A D Seg is the most common bifocal used. It is a sideways “D” shape which gives the widest field of view at the topmost point just as the eye is starting to look down for reading. It has the disadvantage that there is a slight ridge at the top so it is a bit more obvious.

Round seg:

The round seg is still very popular. The area of reading vision is less than that of a D seg but it has the advantage of being less obvious.

Curve top:

This is a kind of hybrid between the D seg and round seg. It has the wider field of the D seg but the flat top of the D has a slight curve applied to give less of a ridge.

Executive:

Executive bifocals have a line straight across the lens. This gives the widest possible reading area but it has a very prominent ridge at the junction between the distance and the reading part. There is one other lens which is worth a mention. This is the “Franklin Split Bifocal”. This is a lens which is made of two separate pieces put into the glasses frame together. The distance lens is completely flat at the bottom and the reading lens is completely flat at the top.

Tags: astigmatism, bifocal, bifocal glasses, crystalline lens, Curve Top, D seg, distance glasses, eyesight, field of view, Franklin Split, Franklin Split Bifocal, hypermetropia, long sightedness, myopia, patient, presbyopia, reading add, Round Seg, rugby ball shaped eyes, short sightedness, vision

This entry was posted on Friday, January 2nd, 2009 at 7:38 pm and is filed under Eye Care. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.