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The lens is made up of a series of layers, and as we get older the thickness of the lens increases in much the same way as a tree trunk gets thicker as it ages. When either of these structures thicken they become harder to bend. You can bend a sapling quite easily, but a few years later it resists bending more. And a few years later still, you can’t bend it at all despite being just as strong.
The same problem occurs with the eye as we age. When we want to bend the lens (we call it accommodating) – even though we still exert as much force with our muscles – we get a lot less of a change in the shape of the lens as it thickens, and so less of a power change. The result is that we start holding books etc. further away so we don’t need as much power. But then the print starts to become too small or our arms aren’t long enough! Even with this adaptation, the eyes are working at their maximum and after a short while feel uncomfortable, lose focus and possibly give headaches. This is the time that reading glasses are needed.
The correct prescription will give the minimum help necessary to make the level of work needed by the eye comfortable, whilst still leaving the eye to make up the rest and also keeping tone in the muscle. Unfortunately, as the eye gets older the lens will become thicker and less pliable meaning a greater dependence on the spectacles. The cause of the problem is not that the power in the muscles has deteriorated, rather that the same amount of force exerted on the lens will no longer create the distortion it once did.
How do we compensate?
The power needed to give a clear near focus is too strong to allow a clear distance focus. So spectacles that are correct for reading will not allow you to see clearly in the distance. And if you need correction for the distance, this power will not be sufficient for reading. We need two different powers for the two circumstances. There are three main options to deal with this:
- Standard contact lenses are not suitable in this circumstance as with this form of correction the distance would be corrected for and the eye is unable adjust the focus for near.
- Varifocal contact lenses are now much improved and often can offer the spectacle free life that was previously enjoyed.
- Refractive surgery with varifocal implants is also an option but generally only for those age 55+.
Single vision lenses
As the name suggests they correct just one power and are the type used when making up just reading or distance spectacles.
Two different powers are incorporated into the one lens. One part of the lens is dedicated to distance vision. The lower part has a definite area (the segment) given over to close work. Segments can be made with different shapes and sizes and are normally discernible when looked at closely.
Again, the top of the lens is dedicated to distance vision and the lower area to close work but there is a gradual transition from one to the other. The reading area in this type of lens is not as wide as with most bifocals but they have no sudden change from one power to the other and have no distinguishing lines on them. There are areas of blurring or distortion at the edges of the reading area and it is necessary to move your head slightly when reading across wide areas.
Some lenses have more marked areas of distortion than others. We will not use the cheapest lenses where this distortion is most marked. We stock a wide range of varifocals where distortion is minimised. We are one of the few opticians able to supply the very latest individually designed varifocals from Rodenstock– they are a premium lens whose performance matches their price.