Spectacle lenses come in all sorts of different styles, shapes, designs, sizes and densities and there may be some lenses that are not suitable for your particular prescription or the tasks for which you are going to use them.
Single Vision or Multifocal ?
One of the first decisions to make about your spectacles is whether they are to be Single Vision or Multifocal.
If you have only a distance prescription, or only a reading prescription, then single vision lenses may be all that is required. However many people with a distance only prescription cannot read with their distance spectacles on, and those with a reading prescription only cannot see the television with their reading spectacles on. It can then be inconvenient to be always taking spectacles on and off which is where a multifocal lens is useful.
Multifocal lenses enable you to see at multiple distances with the same pair of spectacles. They can be bifocal, trifocal or varifocal and the focussing distances can be any combination of distance vision (television), intermediate(computer) or reading.
Bifocal lenses have two distinct portions of the lens for two different focussing distances. They can be for Distance and Reading, Distance and Computer, or Computer and Reading.
Trifocal lenses have three distinct portions of the lens for three different focussing distances – usually Distance, Computer and Reading.
Varifocal lenses do not have any distinct portions visible on the lens – it looks just like a normal single vision lens with no lines on it, but the power of the lens gradually increases the lower down the lens you look. This means that whilst looking straight ahead Distance things are in focus. If you tilt your eyes down slightly, nearer things come into focus such as the Computer screen, and if you look further down the lens the focus has shifted even closer to be comfortable for Reading.
The latest addition to lens design are ‘office’ lenses. These degressive lenses are ‘clever reading glasses’ for all the near vision tasks of the 21st century – computer screens, laptops and mobile phones – as well as traditional book and newspaper reading. They are the ideal solution that enable you to have one pair of spectacles for all your near vision tasks.
All the lens types listed above can be made thinner, or thicker, lighter or heavier depending on the lens material that is used.
The amount by which a spectacle lens focusses the light is determined by the refractive index of the lens and its curvature.
A standard lens has a refractive index of 1.49 but there are lenses available with refractive indexes of 1.60, 1.74 and even 1.90 which enables the edge thickness of your spectacles to be reduced by as much as 50% if you have a strong prescription.
The disadvantage of using a lens with a higher refractive index is that the lens is subject to more aberrations or imperfections in the way that the light passes through the lens.
Another way of reducing the thickness of a lens is to make the lens surface not spherical but aspheric in shape. This is how varifocal lenses change their power, but it can also be used on any design of lens to reduce the centre thickness of the lens making it cosmetically more appealing and also lighter in weight.
The disadvantage of aspheric lenses is that they can reduce the amount of magnification perceived by the eye which can reduce overall visual performance.
It is important therefore to achieve a balance between refractive index, aspheric lens thickness and optical performance by careful assessment of your prescription to ensure the optimal performance of your individual lenses.