Ophthalmic lenses bend light to correct focusing defects of the eye. Lenses that are thicker in the middle and thinner at their edge converge light and can be used to correct hypermetropia. Lenses that are thinner in the middle and thicker at their edge diverge light and can be used to correct myopia. Lenses that correct astigmatism have a non-rotationally symmetric surface with unequal curvatures in different meridians.
The optical power of a lens refers to its effect on light passing through its central portion. At points distant from the centre of a lens there are unintended 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. Not all lenses allow for equally good vision. 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 low powered straight-forward prescriptions. Large optical prescriptions are particularly susceptible to aberrations and so high quality lenses are more critical when these are necessary. There is necessarily an increased cost with rising lens quality because they are not ‘off-the-shelf’ items that can be mass-produced. These lenses have complex surface geometries that are optimised individually, rather than being an average for a range of prescriptions.
The performance of all ophthalmic lenses depends on how they are positioned before the eye: horizontally, vertically, tilt and position from the eye. An optometrist or dispensing optician can offer advice you on the best lens for your based on your spectacle prescription, expected usage and frame choice, and take accurate measurements to ensure optimal vision.
The College of Optometrists in their guidance on best practice note that the prescribing and dispensing of spectacles are very closely linked and so recommend that it is in the best interest of patients to have spectacles dispensed where the eyes are examined.
Ophthalmic lenses were traditionally made of glass. This has excellent optical properties and superlative resistance to scratches. However, it has fallen out of favour in the last 20 years due to concerns with safety and the improvement in plastic lens manufacturing. Indeed, now plastic lenses give equivalent or even better optical performance because they can be made with more complex surface designs.
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 allow for more attractive lenses that are lighter, and so are much more comfortable to wear.
Types of ophthalmic lens
These are lenses have the same power across the lens. If they are powered to give clear distance vision in children and young adults, they also allow for clear near vision because additional focussing power can be supplied by flexing of the internal crystalline lens. Presbyopia develops in middle-age and beyond that causes the crystalline lens becomes rigid and unable to change focussing power. It follows, that after the onset of presbyopia single vision lenses cannot provide clear vision at both distance and near, and so separate pairs of spectacles or bifocals/varifocals are required.
These lenses offer correction for both distance and near vision. The near correction is provided by a segment at the bottom of the lens.
Bifocals give wide undistorted vision in both areas of the lens, and are optically the best solution for presbyopia. However, bifocals are not liked by some people due to the visibility of the reading segment that creates a line that is visible to both the wearer and observers. Also, while offering unsurpassed quality of vision at both distance and near they lack an intermediate focus, which is increasingly important in our computer age.
These lenses provide a gradual increase in focussing power with a progressive increase in curvature moving down one of its lens surfaces. These lenses provide a focus for all viewing distances, and have no physical line, but owing to the necessity of blending different curvatures at the edges of the lens have peripheral distortions that limit the width of the central ‘corridor’ of clear vision.
Improvements in varifocal design and manufacturing techniques have allowed for huge improvements in the functioning and tolerance of these lenses such that most people can wear them for most things. Notwithstanding these technological advancements, the width of clear vision is necessarily narrower than with single vision or bifocal lenses. The compromise in vision with varifocal is most noticeable for near and particularly for intermediate vision. It is for this reason that varifocal wearers must learn to move their heads more than their eyes, and people with critical visual requirements or who use a computer or read for long periods often do best with an additional pair of spectacle with an alternative type of lens for these activities.
These lenses have been developed in response to the limitations of varifocals with providing comfortable vision for intermediate and near tasks. The top portion of the lenses give clear vision for intermediate distances and the bottom portion of the lens provides clear vision for near tasks.
Office lenses are similar to varifocals in many ways in that they achieve this with a lens surface that has a progressive increase in curvature move down the lens. They are able to provide a greater width of vision than varifocal because the difference in power between the top and bottom of the lens is much less.
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 coating 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.