Spectacle Information
Different Types of Spectacle Lenses
Single Vision
Single Vision lenses have only one power. For short sighted people these are negative lenses (thinner at the centre) and for long sighted people positive lenses (thicker at the centre).
Single Vision lenses have only one power. For short sighted people these are negative lenses (thinner at the centre) and for long sighted people positive lenses (thicker at the centre).
For higher prescription glasses for short sighted people (those with myopia) the edges of the lenses start to become quite thick and unsightly and for long sighted people the lenses become thick in the centre. To improve cosmesis and to reduce the weight of the lens a higher index lens (which will give a thinner and lighter lens) can be used. The dispensing optometrist can show you how much thinner your lens will be. For high power lenses choosing an aspheric lens will also help reduce the centre thickness for positive lenses and improve off axis (when not looking through the centre of the lens)
For presbyopic people single vision glasses will only provide clear vision at one distance - therefore it is necessary to have one pair for distance, one pair for near and possibly one pair for intermediate distances (for the TV or computer), however some lucky short sighted people can take their glasses off to read.
The advantage of having separate pairs is clear comfortable vision at each distance but the obvious disadvantage is that the glasses have to be changed for different tasks.
Bifocal
Bifocal lenses have two powers - usually a distance portion that contains a lens with the distance prescription and a near area - that contains the prescription for close work.
A Bifocal Lens
The reading area can have different shapes - for example round or shaped like a D. The reading segment is visible on the lens.
The advantage of bifocals is that they give distance and near vision with no distortion. The disadvantages are that the segment is visible and hence is not cosmetically pleasing, the lens doesn’t give any intermediate vision - e.g. for the computer and as the eye moves into the reading area the image appears to jump.
Varifocal
Varifocal lenses are designed by computer programs to give a lens that gives vision in the distance, intermediate and near with no visible segments. The power of the lens changes from the top of the lens to the bottom with the distance area at the top, the intermediate in the centre and the reading area at the bottom.
Hence to look through the right area of the lens the head needs to be moved rather than the eyes - for instance for reading the chin must be tilted up so that the eyes look through the lower part of the lens. Most people get used to this in time and enjoy having vision at all distances rather than having to change specs or wear an unsightly bifocal lens.
Making a lens that gives vision at all distances is difficult and in order to achieve this some distortion will be seen when not looking through the centre area of the lens. This means that in order to look left and right the head needs to be moved rather than the eyes to keep clear vision.
Making a lens that gives vision at all distances is difficult and in order to achieve this some distortion will be seen when not looking through the centre area of the lens. This means that in order to look left and right the head needs to be moved rather than the eyes to keep clear vision.
Occupational
Occupational lenses are similar to progressive lenses, however instead of being set for distance, intermediate and near they are set for intermediate and near. This is very useful at work when your tasks include reading and looking at a computer simultaneously, the intermediate range can be up to about 10 feet.
These lenses are significantly cheaper than progressive lenses allowing an affordable lens to use at work especially if you do not need glasses for distance vision.