Lens types
Single vision – has the same optical correction over the entire area of the lens.
Bifocal – the upper part of the lens is generally used for distance vision, while the lower part is used for near vision. Usually, a segment line separates the two. Typically a person with myopia would have one section of a prescription lens that has a certain diverging power while another section of the lens would have a lower diverging power for close-up work. Similarly a person with hyperopia would have one section of the lens with a certain converging power and another section with a greater power for close-up work.
Trifocal – similar to bifocals, except that the two bifocal areas are separated by a third middle area with intermediate correction, used for intermediate distance vision. This lens type has two segment lines, dividing the three different correcting segments.
Progressive or varifocal – provide a smooth transition from distance correction to near correction, eliminating segment lines and allowing the viewing of all intermediate distances.
Adjustable focus– dynamically adjust focus to enable viewing over the entire lens area at all distances from near to far and everywhere in between.

Lens shapes
Although corrective lenses can be produced in many different shapes, the most common is ophthalmic or convex-concave. In an ophthalmic lens, both the front and back surface have a positive radius, resulting in a positive / convergent front surface and a negative / divergent back surface. The difference in curvature between the front and rear surface leads to the corrective power of the lens. In hyperopia and presbyopia a convergent lens is needed, therefore the convergent front surface overpowers the divergent back surface. For myopia the opposite is true: the divergent back surface is greater in magnitude than the convergent front surface.

Bifocals and trifocals result in more complex lens shapes. A bifocal adds a second lens called an add segment to a standard distance corrective lens. There are many shapes and sizes of segments and the method that they are combined with the distance corrective lens has to do with the lens material and segment type (shape).

Progressive lenses, which eliminate the line in bi/tri-focals, are very complex in their shape as they are no longer the combination of two spherical surfaces.

The base curve (usually determined from the shape of the front surface of an ophthalmic lens) can be changed to result in the best optic and cosmetic characteristics across the entire surface of the lens. Optometrists may choose to specify a particular base curve when prescribing a corrective lens for either of these reasons. A multitude of mathematical formulas and professional clinical experience has allowed optometrists and lens designers to determine standard base curves that is ideal for most people.

Lens Coatings

Lens coatings can enhance the performance and appearance of eyeglass lenses.

Scratch-Resistant Coatings
No eyeglass lens material - not even glass - is scratch-proof. However, a lens that is treated front and back with a clear, hard coating does become more resistant to scratching. Nowadays, most types of plastic lenses, including high-index, polycarbonate and traditional plastic materials, have built-in scratch-resistant coatings. Since it is sometimes optional, make sure your optician knows that you want scratch-resistant lenses in your prescription eyeglasses.

Ultraviolet Treatment
Another lens treatment that is beneficial but invisible to the naked eye is ultraviolet (UV) protection. Just as we use sunscreen to keep the sun's UV rays from harming our skin, UV treatment in eyeglass lenses blocks those same rays from damaging our eyes. Overexposure to ultraviolet light is thought to be a cause of cataracts, retinal damage and other eye problems.

An ultraviolet treatment is simple and quick to apply to most plastic eyeglass lenses, and it does not change the appearance of the lenses at all. The exception is polycarbonate lenses, which don't need anti-UV treatment because it is an inherent property of the material.

Anti-reflective Coatings
To improve both the vision through the lenses and the appearance of the glasses, an anti-reflective coating (also called AR coating) is applied. AR coatings are similar to the coatings found on microscopes and camera lenses. They consist of several layers of metal oxides applied to the front and back lens surfaces. Because of the layering effect, AR coatings sometimes have a hint of green or purple color, depending on the individual manufacturer's formula.

Each layer is scientifically calculated to block reflected light. The result is that you'll see a reduction in glare, annoying reflections and halos around lights. This is a great safety benefit when you're driving at night.

Also, anti-reflective coating reduces both internal and external reflections on the lenses themselves, creating a nicer cosmetic appearance. Internal reflections appear as rings that make lenses look thick. External reflections mask your eyes from a clear, complete view when someone is looking at you. So with an anti-reflective coating, eyeglass lenses appear thin or non-existent, and your eyes look more natural.

With sunglass lenses, an AR coating is better applied only to the back surface of the lens (the surface nearest the eye). Because sunglass lenses are so dark, the AR coating can wind up looking smeary on the front surface. Coating the back side helps reduce the reflections of light that enter from behind you and bounce off the surface into your eyes. A back-side coated sun lens is much more comfortable than an uncoated sunglass.

Mirror Coatings
In contrast to anti-reflective coatings, which are very clear, mirror coatings (also called flash coatings) are bold statements of color. Just as the name implies, a mirror coating is highly reflective. The mirrored sunglasses associated with state troopers are one example of a flash coating.