Lens material: This will determine the clarity, weight, durability and cost of your sunglasses.

Glass: "These lenses offer excellent clarity and are the most scratch-resistant of all materials, but are heavier and can break," says Shailesh.
Polycarbonate: The combination of light weight, impact resistance and low bulk make this a popular choice.
Acrylic: This material is tough and a more affordable alternative to polycarbonate. Although it offers less optical clarity, it is useful if you seldom use your sunglasses or as a temporary replacement pair.

Lens colour (tint): Used to reduce overall brightness, but different colours can have different effects on your vision.

Grey lenses give true colour perception. Grey is a popular neutral colour and a good general purpose tint, although it does not enhance contrast. It is a good tint for cycling or running.
Green also gives true colour perception and is a popular neutral colour. It's a good general purpose tint, giving fair contrast in low light. It also lessens eye strain in bright light.
Brown is better in hazy sun conditions, as it enhances contrast. A good tint for high-glare sports such as skiing, fishing or sailing.
Amber blocks blue light. It brightens cloudy, hazy or foggy days. Through this tint, the world looks yellow or orange. It is excellent for contrast and minimises eye strain. A good tint for sportsmen, pilots and skiers.
Vermilion provides excellent depth perception in low light conditions. As it contrasts objects against blue and green backgrounds, it is a good tint for sportsmen.
Rose provides excellent low-light visibility and enhancement. A good choice for water sports or other outdoor activities, due to the good contrast it provides for objects viewed against blue or green backgrounds.

Frame: Usually made of metal, nylon or plastic.

Metal: "These are strong, but not flexible, and can break or bend permanently if you twist them," says Shailesh.
Nylon: Often used in sports sunglasses -- lightweight, strong and flexible.
Plastic: Lightweight, strong and flexible. "Choose lightweight, plastic, shatterproof sunglasses if participating in any sport," advises Shailesh.

Hinges: "Less expensive sunglasses usually have hinges molded right out of the frame material, which can result in less durability. Higher-quality frames have durable nickel-silver hinges. Spring hinges are used on some frames, allowing them greater bending ability than regular frames," says Shailesh.