Spec lens designs
Spec lens design can appear quite complex but I will try and simplify it.
This is when you only need a single focus lens i.e. for reading or driving. They are the simplest form of lens design.
This is a lens that combines two different focuses, usually distance and reading, onto one lens. They are also very useful when watching television and also reading or knitting at the same time as both will be in focus. I can adapt these to make them excellent lenses for golf or cycling.
These blend from the distance power at the top of the lens to the reading power in the lower part of the lens. Because of this 'blending' the middle distance is also in focus in the central area of the lens. These lenses are brilliant for general wear, people who work with computers or anyone who gets fed up of taking their specs on and off. There are many different types of varifocals and you should always be aware of which ones will suit you.
These are lenses specifically designed for those of us that work with computers. They range from close reading vision at the bottom of the lens to computer vision at the top. These can be individualized to your working distance.
All lens materials have a refractive index number. This number indicates the materials ability to refract light waves and the higher the number the less material is needed to produce the power of the lens. This results in a thinner lens. So a high -power lens will be thinner at the outer edges whilst a high +power lens will be thinner at its centre.
CR39 plastic 1.5
The most commonly used lens material
A thinner, tougher lens material used a lot for safety and sports specs.
Mid index 1.67
Very popular material as it is cosmetically and optically superior to CR39 but not too expensive.
High index 1.74
Thinner still than 1.67 Produces a cosmetically pleasing lens for the higher prescriptions.