Multifocal or progressive lenses are like the name implies, lenses with a multitude of focuses and progressive powers. The lens looks like a single vision lens, without any dividing lines or sudden power changes and offers various focusing powers. The lens has distance, intermediate and near zones, which allows for clear vision at all distances.
Although peripheral distortions exist within the lens, this lens remains the best visual compromise to provide clear vision at all distances. These lenses are available in a variety of tints and materials. They are also able to accommodate all the popular lens coatings that offer the best performance and comfort for the wearer With progressive lenses, you can look up to see clearly across the room and in the distance. You also can look ahead to view your computer in the intermediate zone and drop your gaze downward to read and do fine work comfortably through the near zone of the lenses.
The popularity of progressive lenses has exploded in recent years, making progressives the most widely purchased lenses for correcting presbyopia. Today there are many progressive lens designs to fit virtually any needs.
For all powers of progressive lenses to fit within a pair of spectacles, frames in the past had to be relatively large. If the frame was too small, the reading portion of the lens would sometimes end up uncomfortably small after the lens was cut to size and inserted in the frame. Lens manufacturers have overcome this problem by introducing "short corridor" progressive lenses with compact designs that provide larger reading zones for today's smaller, fashionable frames.
The differences in lens design are related mainly to the length and width of the progressive power corridor and how much of it is devoted to different viewing distances. Different areas of the corridor may be expanded, depending on the design philosophy of the manufacturer and the intended purpose of the lens.
Some progressive lenses are made specially for computer use, for example, and have a wider intermediate zone. Other progressive lens designs may have a larger reading portion. Your eye care practitioner will prescribe the lens style that will best suit your needs.
There is always an Adaptation period for progressive lenses, especially if it is ones first pair. Minor peripheral distortions are unavoidable in progressive lenses. It is impossible to create a seamless (line-free) multifocal lens that has multiple powers for different viewing distances without also creating unwanted distortions somewhere in the lens.
Lens designers and manufacturers have made significant strides in minimizing these distortion areas and "pushing" them to the periphery of modern progressive lenses. But peripheral distortions will be present even when progressive lenses are flawlessly produced using the latest manufacturing equipment and processes — they are an unavoidable optical limitation of all progressive lenses. If you glance to the far right or left, especially when looking down, you might notice your vision is slightly blurred. Peripheral distortions also might cause you to experience a sensation of "swim" when you make quick head movements. It is normal for you to experience these problems when you start wearing a new pair of progressive lenses. You can usually eliminate them by making slight head movements to look more directly at objects. A simple rule is to point with your nose where you want to see, and then slowly lift your chin, to find the clear spot on the lens that is suited to your viewing distance. Try to avoid moving closer or further from the object you are looking at, but rather move your chin up or down slowly to find the position on the lens that is suited to that distance.
Most people who notice peripheral vision problems when wearing progressive lenses find that these issues are relatively mild and disappear as they adapt to wearing the lenses over a period of a few days. Be fair to yourself by giving yourself a few days to adapt to your new lenses
These lenses are available in a variety of materials including Glass, and a variety of different Plastic materials with different optical properties. Barium Crown Glass is the glass that has been used for most lenses ranging from microscopes, telescopes, binoculars, cameras and spectacle lenses. Higher density flint Glass is also used as it has a higher refractive index than crown glass, but due to the higher density of the material the lenses are heavier.
Technological and chemical advancements have produced Plastic materials that are well suited to spectacle lenses The plastic material most commonly used is known as CR39 ( "Columbia Resin 39"). It is a thermal-cured plastic developed in the early 1940s. CR-39 plastic remains a popular material for spectacle lenses because of its light weight (about half the weight of glass), low cost and excellent optical qualities.
Polycarbonate Material developed in the early 1970s, is also used due to its incredible impact resistance. Polycarbonate lenses became increasing popular and remain so today. This material is used for helmet visors for the Air Force, for "bulletproof glass" for banks and other safety applications, polycarbonate is lighter and significantly more impact-resistant than CR-39 plastic, making it a good material for children's eyewear, safety glasses and sports eyewear.
A newer lightweight lens material with similar impact-resistant properties as polycarbonate called Trivex was introduced for eyewear in 2001. The advantage of this material is its superior optical properties compared to polycarbonate.
The refractive index of lenses refers to the light bending properties of the specific material. The more “dense” the material, the higher the refractive index. For example, Air has a refractive index of 1. And pure water has a refractive index of 1.33, and the densest clear material known to man is a diamond, which has a refractive index of 2.4. the higher refractive index also means the specific material reflects light more, as the light cannot enter it easily due to the increased density of the material, that is why diamonds “sparkle” as they do because they reflect the light more.
The Refractive index of Glass ranges from 1.53, to 1.9. The Flint Glass which has the higher refractive index Is heavier than normal glass due to the higher specific gravity of the material. In Plastic, the refractive index ranges from 1.498 (CR-39 plastic) to 1.74 (a specific variety of high-index plastic). So, for the same prescription power and lens design, a lens made of CR-39 plastic will be the thickest and a lens made in 1.74 index plastic will be the thinnest.
The higher index lenses on the downside tend to reflect light more, and for this reason it is essential to treat the surfaces of these lenses with an Anti-reflective coating.
Tints of virtually any color can be applied to lenses. Lighter, fashion tints are often used for cosmetic purposes to enhance a wearer's looks. Darker tints allow the wearer to use the lenses as sunglasses. Color can be added to a lens as a solid tint, where the entire lens has the same color density, or as a gradient tint, where the color density is darkest at the top of the lens and gradually fades to clear or nearly clear at the bottom.
Different colors can be applied to lenses for different purposes. Yellow is often added to a lens to enhance contrast, especially in overcast conditions, making it a popular tint for hunters and for driving at night, but specifically during the twilight times just after sunset, and just before sunrise. Green, Green/grey, Grey, and Brown tints used in varying densities depending on user preference, are the most popular for sunglasses.
Various slight shades are sometimes added to lenses where the user needs them for a specific purpose. Some users find relief from a slight tint when working in an office, specifically when using a computer most of the day. Gradient tints are popular as the top portion is slightly tinted to reduce bright light in the distance portion, and the lower reading, or near work area is left clear.
Cumulative exposure to the sun's harmful ultraviolet (UV) radiation over a person's lifetime has been associated with age-related eye problems including cataracts and macular degeneration. For this reason, people should protect their eyes from UV beginning in early childhood.
An ultraviolet protective coating can be applied to plastic lenses to block out the harmful Ultraviolet rays. Polycarbonate and nearly all high-index plastic lenses have 100 percent UV protection built-in, due to absorptive characteristics of the lens material. Anti-reflective coating (also called "AR coating" or "anti-glare coating") improves vision, reduces eye strain and makes your spectacles look more attractive. These benefits are due to the ability of AR coating to virtually eliminate reflections from the front and back surfaces of your lenses. With reflections gone, more light passes through your lenses to optimize your vision with fewer reflections and ghost images (especially at night), the lenses look nearly invisible. Apart from the optical benefits, this enhances your appearance by drawing more attention to your eyes and helping you make better "eye contact" with others.
All lightweight plastic lens materials have surfaces that are significantly softer and more prone to scratches and abrasions than glass lenses. Polycarbonate is the softest lens material, but is also the most impact-resistant. All plastic and high-index plastic lenses require a factory-applied anti-scratch coating for adequate lens durability. Most of today's modern anti-scratch coatings (also called scratch coats or hard coats) can make your lenses nearly as scratch-resistant as glass.