Ophthalmic lenses bend light to correct focusing defects of the eye. The optical power of a lens refers to its effect on light passing through its centre. At points distant from the centre of a lens there are deviations in its optical power, termed aberrations, which degrade vision and restrict the usable area of the lens. We do not only look through the middle of a spectacle lens and so it follows that for comfortable vision any lens must be centred accurately in front of the eyes and have a design that minimises off-axis aberrations. High quality ophthalmic lenses afford better aberration control, and so provide more natural vision than low-grade alternatives.
The advantages of high quality ophthalmic lenses are often marginal for straight-forward prescriptions, and when this is the case your optometrist will not recommend a more sophisticated product than is needed. Large optical prescriptions and progressive lenses (varifocals) are particularly susceptible to aberrations and so high quality lenses are more critical when these are worn. There is necessarily an increased cost with rising lens quality because they are not "off-the-shelf" items that can be mass-produced using moulds. These lenses have complex surface geometries that are optimised individually, rather than being an average for a range of prescriptions, and are cut using a precise computer controlled lathe.
Moderate and high powered lenses are characterised by a thick edge or a bulbous lens form that magnifies the eye when used to correct myopia and hypermetropia, respectively. Ophthalmic lenses can be made thinner and flatter by using materials that have a higher refractive index and/or aspheric lens surfaces, which gives you better looking spectacles that are lighter, and so are much more comfortable to wear.
Anti-reflection lens coatings improve both the vision you experience through your lenses and the appearance of the lenses themselves. At least 8% of light is reflected with uncoated lenses, which makes glare worse when driving at night and is distracting for people looking at you because they do not have a clear view of your eyes. Modern coatings also improve resistance to scuffing and smudges, and repel water and dust.
Early anti-reflection coatings were plagued by crazing because they consisted of a single layer bonded to the front of a lens that expanded at a different rate with variations in temperature. Nowadays, an anti-reflection coatings is composed of many layers, each less than a hundredth the width of a human hair and deposited within submicron tolerances by condensation in a vacuum.
A fixed tint that is uniform across the whole lens can incorporated by soaking a lens in a heated dye.
Alternatively, a liquid resin containing a dye that becomes darker in the presence of ultraviolet (UV) radiation can be applied to the surface of a lens and polymerised. These photochromic lenses are excellent for people with an outdoor lifestyle because they negate the need to regularly switch between clear and tinted spectacles. However, they are not a substitute for driving sunglasses because they do not react well behind a car windscreen that blocks the necessary UV.
Zeiss have produced some of the finest lenses in the world for more than 160 years, and remain at the forefront of modern lens design. Their lenses guarantee the highest level of visual comfort.
The company manufacture aspheric lenses and inner surface progressive lenses that reduce peripheral distortion with flatter, more attractive lenses. They also lead the industry with their coating systems that increase the hardness of plastic beyond glass, reduces over 99% of reflections that cause glare, and incorporates surface treatments that create an anti-static smudge-resistant barrier so lenses stay clean longer.
Essilor are the world's largest ophthalmic lens manufacturer that invest in R&D and are renown for their innovative products. They developed the first progressive lens (varifocal) in 1959 and have their UK production facilities in Thornbury.
Many of the staff at Essilor have their eyes examined at Johnson & Furze Optometrists and Opticians.