The primary component that brings about clearer eyesight is nothing more than the lens. Most people are interested in picking out the most beautiful frame, but please keep in mind that you have to be careful in selecting the lens as well. There are 3 main characteristics of lens you have to consider: type of lens, lens design and raw material of the lens. We can start by discussing about the types of lenses available and lens design first, and talk about lens’ raw material later.

The types of lenses which are suitable for different eyesight problems are as follows:

• Single vision lens
• Bifocal Lens
• Progressive lens

Single Vision Lens

This lens has single lens power (supports only one focal length). The single vision lens can be divided into three types based on the eyesight problem:

• Concave lens - this is for nearsightedness. Looking at the picture, you will see that the edge of the lens is thicker than at the center. For the same lens power, the bigger sized lens you have, the thinner the lens gets. For strength the best cutting is to use big lens together with small frame.    

 

• Convex lens – this is for farsightedness and the aging eye. The edge of the lens is thin and becomes thick at the center. For the same lens power, the bigger lens you have, the thicker lens you will get. The best thing for weight then is to use smaller lens.
• Cylindrical lens – this is for Astigmatism. The edge of lens is not the same thickness. For people who have both nearsightedness and astigmatism, manufacturers can create concave cylindrical lens for this purpose. Convex cylindrical lens can also be created for a combination of farsightedness and astigmatism.

Single Vision Lens Designs

The single vision lens has two basic designs: spherical an aspheric. The spherical lens is the original design on which both side of lens surface are created with a circular curve. The problem of this lens is the distortion at the lens’ edge, especially for a high power lens. Aspheric lens are developed to correct the distortion at the edge of the lens. This type of lens is flatter and thinner. Each manufacturer develops varieties of aspheric lens models to match a specific eyesight problem.

Bifocal Lens

This lens is used for the aging eye. A bifocal lens has two different focal lengths. A small reading zone at the bottom and another focal length for the distance sight requirements. So the wearer can see both distances clearer in one eyeglass. There are four types of bifocal lens according to the shape of the reading zone as follows:

• Flat Top – the reading zone is in a half circle shape.
• Krypt Tok – the reading zone is in a circle shape
• Executive – the reading zone and distance zone are in each half of the lens. The joint can be very obvious so this lens does not look very nice, but are suitable for persons who need bigger reading zone.
• Crystal B – the reading zone is in circle shape and the joint is more seamless. 

However, a bifocal lens is typically not very aesthetic because we can see the difference between two areas easily. And it cannot serve a variety of focal lengths for different sight distances. A bifocal lens is however easy to use and easy to dispense compare to progressive lens and of course the price is typically much cheaper.

Since there are a lot of details to cover about progressive lenses – the most complex lens – we will discuss these in another chapter. Don’t forget to read!