Single vision spectacle lenses are used to correct vision problems including short-sightedness (myopia), long-sightedness (hyperopia), and astigmatism. In all of these cases light passing through the eye is being projected away from the part of the retina needed to provide clear vision, which results in blurriness. To correct for this, lenses are shaped to re-direct and focus the light towards the centre of the retina to allow clear, crisp vision.
As we get older, our eyes can begin to have difficulty with reading as well as distance vision. A simple solution to this problem is to have one pair of glasses for distance vision and a second pair for reading. But simpler still is to get bifocal spectacles. Bifocal lenses are distance vision lenses with a discrete reading segment added to the bottom of the lens. This type of lens allows the wearer to see clearly for both distance and reading while affording the convenience of having a single pair of spectacles.
A step up from bifocal lenses are progressive lenses, also known as varifocals. The difference between these lenses is that while bifocals have a discrete line between the distance and reading areas, progressive lenses have a gradual, non-visible transition from distance to reading. The graduation between distance and reading in progressive lenses is achieved by ‘surfacing’ the lens; That is, grinding down particular areas of the lens to different thicknesses depending on the desired effect. The benefit of progressive lenses is that they allow you to view objects at intermediate distance, such as your computer, as well as achieving excellent distance and reading vision.
Photochromic lenses are lenses that change from being clear while indoors to dark when outside. This removes the inconvenience of having to switch between spectacles and sunglasses throughout the day, and ensures your eyes are always protected against harmful UV rays from the sun while outdoors.
Having a high prescription can mean that spectacle lenses are thick and heavy. To prevent this, your spectacles can be made with high index lenses that are thinner and flatter than standard lenses, making your spectacles more comfortable while providing the necessary level of visual correction. High index lenses are made of materials which bend light at a greater angle than standard lenses, allowing the lens to be thinner. As well as improving comfort, high index lenses also help to reduce minification and magnification of the appearance of your eyes through the spectacles lenses when wearing a high prescription.
For more information about how different types of lenses work, visit Essilor’s website here: http://www.essilor.co.uk/all_about_vision/lenses