Time can gradually take a toll on one’s body, and whilst some effects of age are obvious, some are less subtle and come on gradually. Some people may notice it getting increasingly difficult to see objects that are close-by, such as during reading or using the computer. Perhaps it has become difficult to read whilst holding the newspaper or book at arms’ length.
Bifocal glasses are prescription glasses that can help people with presbyopia – a difficulty with seeing both near and far away objects. eBay hosts a range of bifocal glasses that can be bought online, once an optician has checked and measured one's eyes, and informed a buyer on the necessary information needed for purchase.
What Are Bifocal Glasses?
Bifocal glasses are a form of eyeglasses frames that bear optically corrected lenses, and are worn in front of the eyes for better vision. Bifocal glasses are distinct from normal prescription glasses because it has two distinct optical powers in each lens. An optical power is the degree to which a lens converges or diverges light in order for the wearer to be able to see in focus.
Usually, the upper part of the lens is used for distance vision while the lower part is used for near vision, with a segment line separating the two. Later models of bifocal glasses have blended lenses, where the line separating the two is not visible, which can be aesthetically more appealing.
The lens can be modified according to what they will be used for – computer vision on the top and reading vision on the bottom being a common combination. They offer a very wide field of clear vision for both parts of the lens, but there is a sudden ‘jump’ between the two sharply defined parts, leading to a gap in the wearer’s overall vision that can take some time for the brain to adjust to.
Construction of Bifocals
Lenses for most glasses are made with either light, shatter resistant plastic in varying degrees of ‘thinness’ or with heavier, scratch resistant glass.
Bifocals have the most convex lenses for close viewing in the lower half of the frame, and the least convex lenses in the upper half. The first bifocal lenses had two separate lenses that were cut in half and combined together in the rim of the frame, forming a single frame. It resulted in a fragile spectacle, and a way to fuse the sections of the lenses together was soon developed by Louis de Wecker and patented by Dr John L. Borsh Jr.
Although there is evidence of prior use, the invention of bifocal glasses is usually credited to Benjamin Franklin, who is said to have gotten tired of having to constantly switch between two types of glasses. He is said to have devised a rudimentary way of having both types of lenses fit into the frame, with the far-sight lens placed at the top and the near-sight lens placed at the bottom of the frame.
Today, bifocals are created by moulding the near-distance segment into a primary lens in a variety of shapes and sizes. The most popular size is the D-segment 28mm wide which offers superior optics, and is the easiest to adapt to for a new user. The ‘Round 38’ seg bifocals can offer a wider field of vision, ideal for avid readers and users who regularly need to concentrate on their near-vision.
The lens segment, or seg, can be placed on the main lens in one of several shapes:
- Half-moon, also called a flat-tip, straight-top or D-segment
- Round segment: the line separating the two parts tends to be less noticeable in this shape. A type of glasses called the ‘invisible bifocal’ has the visible line entirely blended out, which can cause some optical distortions
- Ribbon segment: a narrow, rectangular area
- The full bottom half of a bifocal lens, called the Franklin, Executive or E-style
Progressive Bifocal Lenses
Sometimes called ‘no-line bifocals’, these are bifocal lenses with all the benefits of having a pair of glasses with double optical powers, without any visible lines and no ‘image jump’. Progressive lenses are also available in trifocal and multi-focal lenses.
Benefits and Side Effects
The bifocal glasses user looks up and through the distance portion of the lens, and down and through the near-sight portion. Generally, and in time, the eye will naturally seek the appropriate section to use for focus.
Whilst greatly reducing the inconvenience of using two pairs of glasses, it may cause headaches and dizziness in first-time users due to the eye continually switching focuses. In most users, this will pass in time, as the brain gets used to it, but it can persist for some wearers.
The reading segment of the bifocals offers a small field of view, with the user having to move their head or the reading material, rather than the eyes to follow the text. This can again take time to get used to. However, bifocal and trifocal lenses offer wider lenses than progressive lenses.
Computer monitors have to be placed directly in front of users for the bifocals, which can lead to muscle fatigue due to the movements of the head – switching to trifocal or mono-focal lenses can help in this case.
Do I need Bifocal Glasses?
Bifocals are most commonly prescribed to people with presbyopia (diminished ability to focus on near objects) who also require a correction for myopia (short-sighted, with objects in the distance being out of focus), hyperopia (long-sighted, with difficulty focusing on objects nearby) and/or astigmatism (optical defect in which it is difficult to focus a point object due to irregular or toric curvature of the cornea or lens).
Bifocal and multifocal lenses are usually prescribed for adults over 40; however, some children and young adults with eye teaming or focusing problems, that cause eyestrain while reading, may be prescribed bifocals. The bottom portion of the lens reduces the amount of strain it takes to read something close-up.
A prescription will always be required for bifocals and near-vision correction and an optician can help determine if you have any of these conditions.
Frames, Styles and Coatings
There are a variety of frames and styles available on eBay, with some manufacturers offering special lens coatings and frame styles. Some features are free and come as standard with some manufacturers, whilst others come with an extra charge. Some of the features that may be available on eBay are listed below:
- Durable, 180 degree flexible sides
- Duoflex hinges that flex outwards and up and down, reducing stress on joints
- High prescription frames to hold thicker lenses
- Superflex sides made from lightweight hi-tech materials
- ltraflex styles can be twisted or contorted and can spring back into shape
- Curled arms help keep the glasses on the face – ideal for children and playing sports
- Rimless frames: the lens portion does not have a frame around them, resulting in a softer, ‘no-glasses’ appearance
- Lens coatings:
- Scratch-resistant: helps minimize surface scratches.
- Anti-reflective coatings reduce reflections and minimizes light scatter. Ideal for driving at night by reducing glare, for prolonged computer use by reducing radiation, and when being photographed. Also reduces ‘power rings’, which are visible rings in high prescription glasses caused by the constant bouncing back and forth of light in the lenses, due to total internal reflection.
- Photochromic lenses adapt to changes in UV light, getting darker when exposed to UV light. Coatings that have 100% UV protection can therefore be used as sunglasses. Car windscreens can filter out 40% of UV rays, reducing the effectiveness of lenses darkening while driving. Also called ‘Transitions’ for the plastic variety, and ‘Reactolite’ for glass.
- Coloured/Ultraviolet: full or graduated coatings that soften harsh or bright light, relieving eyestrain.
- Polarising: eliminates horizontal glare from the sun that reflects off smooth surfaces. Also filters out UV light, giving improved vision and comfort in daylight. Ideal for daytime driving.
- UltraDriveDay: some manufacturers make lenses specifically designed for drivers, blocking out UV light and up to 95% of blue light, which helps increase contrast, making colours appear sharper.
- Tints: purely cosmetic application of colour onto the lenses.
- Anti-static coating: aids lens cleaning.
Bifocal tinted sunglasses can also be bought, which provide protection against the sun’s rays and UV radiation.
A whole range of bifocal glasses is available on eBay to suit every circumstance. With the help of this guide and eBay’s extensive collection, finding a pair of bifocals to perfectly suit the frame, eyesight, requirements and style of the wearer can be quick, simple and easy. Whether it is a new user shopping for bifocals for the first time, or a regular bifocal wearer looking to change their look or style, they won’t have much difficulty finding a pair of bifocal glasses on eBay that will suit their need.