Single vision lenses are lenses that correct vision at one distance only, and are the most common glasses used by patients under 40 (or those who have not yet developed presbyopia). For most people, these lenses are used to clear vision at distance, although reading-only glasses are also an example of single vision lenses.
Famously invented by Benjamin Franklin, bifocal lenses provide two vision zones separated by a visible line. The bottom segment (often “half-moon” in shape) corrects for near or reading distances while the remainder of the lens corrects for distance vision.
Unlike bifocals that correct for near and distance only, trifocal lenses incorporate a third zone into the lens to clear intermediate distances (think computer screens- they are often closer than distance, but farther than reading distance). These three vision zones are separated by visible lines. The upper area is for distance vision, the middle segment provides intermediate vision for distances between 2 and 7 feet, and the bottom segment holds the reading prescription.
Progressive Addition Lenses (No-Line Bifocals)
Also referred to as “no-line bifocals”, these lenses are similar to trifocals in that they have 3 different prescription areas for near, intermediate, and distance vision. Progressives differ from bifocals and trifocals in that the different zones are not separated by visible lines, but rather offer a natural, “progressive”, transition between the different powers on the lens. This eliminates the problem of “image jump” associated with lined multifocals and presents a more attractive appearance.
Occupational lenses are typically bifocal lenses with customized placement of the near zone correction. Plumbers, electricians, and others frequently engaged in overhead work often find double-segment lenses preferable, which add a near zone in the top of the lens. Golfers typically experience blur when teeing-off in traditional bifocals, so many prefer a special golf lens which places the near zone into the upper right corner.
Oldest lens material (although archeological findings have discovered emerald and ruby lenses in ancient cultures- we’ll just stick to modern times). Has superior optical qualities (clarity) to plastic lenses, yet are very heavy and are a safety concern due to shattering into small sharp fragments when impacted.
Basic Plastic (CR-39)
Like the name implies, these lenses are, well, basic. The cheapest lens available, but prone to shattering upon impact, and chipping when placed into a drill-mount or semi-rimpless frame.
The most impact-resistant material available- these lenses exceed ANSI safety standards at less than 1/3 the required thickness! Polycarbonate lenses also have the added benefit of being lighter and thinner than basic plastic lenses. Recommended for safety, sports and children’s glasses.
Trivex has optical qualities (degree of clarity) nearly identical to glass. Due to manufacturing processes, it is even less likely to crack than polycarbonate, although it is less impact resistant (more prone to shattering than polycarbonate, yet less likely to crack? strange but true- anyways, both are much safer than basic plastic or glass). Recommended for drill-mounted frames.
Hi-index lenses fall between basic plastic and polycarbonate in their degree of safety, but offer the benefit of creating the thinnest lens possible. Hi-Index lenses include 1.60, 1.67, 1.70, and 1.74. Basically the higher the number, the thinner the lens (but also the higher the cost).
While no coating can prevent scratches, an anti-scratch coating, as the name implies, can make your lenses more scratch resistant. Recommended for eyeglass wearers who work in dusty environments to increase the life of the lenses.
Anti-Reflective Coating (anti-glare coating)
For people who wear glasses, distracting reflections and glare can rob the eyes of the up to 14 percent of available light. AR lenses allow virtually all the available light to pass through the lens for sharper, clearer, more comfortable vision. Recent tests prove that people are able to see more clearly with AR lenses, especially under low light conditions like driving at night. Key benefits of AR lenses include: glare reduction, better night vision, better computer vision (especially when using LCD monitors), better lens cosmetics, and reduced eye fatigue. Eyes on 34th Eye Clinic highly recommends anti-reflective coating for all eyeglass wearers, and highly recommends the Crizal Avance with Scotchguard brand as the most superior coating available.
Transitions Lenses change from clear to dark, & block 100% of harmful UV rays. The newest generations of these lenses darken and lighten faster than ever before, and are available in most every lens material. It is useful to know, however, that Transitions lenses will not darken in the car, as they react to UV light, and our windshields in North America have been treated with a UV filter.
Polarized lenses improve vision, especially in strong light conditions. They do this by controlling which light rays are allowed to enter the eye. By filtering-out reflected and scattered light, vision is crisper, and eye fatigue is reduced, as the visual system doesn’t have to “search” through a glared image. Polarized lenses will protect you from UV rays, glare and intense light.
UV Coating, also called a UV filter, is a bath treatment that effectively gives lenses the ability to block 100% of harmful ultraviolet light rays.
Mirrored coatings reduced glare and brightness as well as provide a dramatic and appealing cosmetic appearance. Traditionally, silver mirror was the most common of these coatings, but today, mirror coatings also come in blues, greens, golds, reds, oranges, and violets, allowing for a more custom match to the frame of your choice.
Frames made of a variety of plastics (nylon, zyl, polycarbonate) are excellent in durability and comfort. The most important thing to consider when looking for a plastic frame is to look for one that fits your nose well, or to have one custom fit to your nose. Plastic frames are also good at hiding lens thickness often found in higher prescriptions. As an added benefit, plastic frames are hypoallergenic- meaning your skin will not react to the material (read Metal Frames to find out more)
Metal frames offer greater adjustability to specific faces and, when paired with soft silicone nosepads, are very comfortable. The most common material used in metal frame manufacturing has traditionally been a nickel alloy, making these frames difficult for persons with nickel allergies (many people do have this allergy- just look for green deposits near the nosepads or temples of your current glasses). Hypoallergenic metals are available, however, and include stainless steel and titanium. Titanium frames are very strong and have a slight “memory” property, making them flexible (although not as flexible as true memory-metals).
Flex-Frames (Memory Metals)
These are the infamous “twist and tie” frames (although most won’t put-up with overly-repeated bending and twisting). Flex frames are excellent for adults or children who are rough on their eyewear. Most are composed of hypoalergenic materials
Semi-rimless frames have always been a sought-after choice that has continued to grow in popularity. These frames have an eyewire on part of the lens, while the remaining part of the lens seems suspended. They are cosmetically appealing, yet usually require a polycarbonate, trivex, or hi-index material to be used in lens manufacturing to eliminate chipping of the lens at the site of lens mounting.
The lightest and least-conspicuous frames available, drill-mount frames have no eyewire around the lens at all. The temples (side pieces) and bridge (middle part that sits on your nose) are actually drilled into the lens to make a complete eyeglass. Trivex is the most compatible lens material with this style of mounting, in order to eliminate lens cracking or chipping at the drill-sites. These frames also come in a variety of metals, although titanium is the most durable, stainless steel is second, and nickel frames are not recommended in this style of frame.
Safety frames come in a variety of materials and designs, most are like their regular eyeglass counterparts, but these meet or exceed ANSI safety standards. Sport frames are an entirely different breed, however, as these frames are expecting to get hit. As such, they are designed to maintain comfort for the wearer even during an impact (when’s the last time you took a basketball to your face and didn’t really mind?). Many of these frames are semi-sport specific, and include racquetball/tennis, basketball, shooting and biking glasses, as well as ski, atv, and paintball goggles. Due to safety concerns, olycarbonate is the recommended lens choice for these frames.