Single vision lenses are used to correct a single refractive need, such as myopia or hyperopia, and may incorporate astigmatic correction where required.
Presbyopes will find their reading glasses blur their longer distance vision, but pre-presbyopes (under 40s) will usually find one pair of single vision glasses adequate for all distances.
A bifocal lens is divided into two segments. Usually the upper segment is designed for distance use and the lower segment for reading.
After the onset of presbyopia, a different prescription is needed for reading from that for longer distance – a bifocal lens combines these requirements into one lens, saving the need to swap between different pairs of glasses for different visual tasks.
Bifocal lenses have a visible line that divides the upper distance portion from the lower reading portion. For this reason, and also because they don’t cater well for intermediate distances, bifocals have been largely superseded by Varifocal/Progressive lenses (see below).
Also known as PALs (progressive addition lenses) or PPLs (progressive power lenses), these have two main advantages over bifocals for the presbyopic wearer:
1) Because the lens blends gradually from the distance prescription at the top to the reading prescription at the bottom, there is no distinct dividing line. Some people find the segment line on bifocals an annoying distraction – others don’t like its cosmetic appearance. No such problems with a varifocal lens – it looks just like a single vision lens.
2) The area between the upper distance and lower reading parts of the lens caters nicely for those ‘intermediate’ distances that bifocals miss out. This makes them more useful for computer work (depending on height and distance of the monitor) and other tasks that require a slightly longer working distance than normal reading.
Varifocals are the most natural solution to presbyopia, though most new wearers take at least a few days (sometimes longer) to get used to them – it’s just a case of learning exactly where on the lens you need to look for each specific distance you wish to focus on.
Presbyopes who wear single vision reading glasses can sometimes feel quite frustrated by the limited focal range of their lenses. Constant adjustment of posture and position to find exactly the right working distance can prove inconvenient and irritating.
Computer and general office work, cooking and decorating – these tasks can all be frustrating for the presbyope. Artists and musicians can also struggle with the varying distances needed for their work. Board games and card games lose their appeal when you’re constantly having to bob backwards and forwards to focus on the play.
Enhanced reading lenses are a relatively new, but very successful solution to the varying visual demands in many similar situations.
Hoyalux Tact enhanced reading lenses provide optimum clarity from 40cm (16 inches) for closer reading, right up to 2m (6 feet 6 inches) away. This is achieved using advanced aspheric surface curvature, which also makes them thinner, lighter and flatter than conventional single vision reading lenses.
They could make all the difference!
If you need a medium to high spectacle prescription there are several ways of making your lenses thinner and lighter than they’d otherwise be.
The ‘strength’ or refracting power of a lens is determined by a number of factors – the combination of front and rear surface curvature, the material used to make the lens, and the resulting thickness of the lens are all telling factors.
By using denser, high refractive index lens materials, the lenses can be made appreciably thinner and a little lighter. There are several choices of high index material in both glass and plastic – most people opt for plastic lenses because they are significantly lighter and safer than glass.
Another means by which lens thickness can be reduced is in using advanced ‘aspheric’ surface curvature. The regular spherical curves traditionally used in lens manufacture can be modified to ‘flatten’ the surface using complex mathematical formulae. This produces an irregular (aspheric) surface that not only reduces lens thickness but can also reduce peripheral aberrations and distortions.
In combining high index materials and aspheric surface curvature it’s now possible to produce certain plastic lens types up to 45% thinner, and glass lenses up to 60% thinner.
The lower picture shows the cosmetic difference between a high index aspheric lens (the left one) and a standard lens (the right one) of the same strength viewed in profile from above.