One pair of spectacles cannot fulfil all visual functions

This is perhaps the most profound statement that can be made about spectacles. Before I elaborate on this statement, let’s take a moment to marvel about our amazing eyes. The retina, which sits at the back of the eye, is a thin layer of tissue that contains millions of tiny light-sensing nerve cells. These nerve cells are called rods and cones because of their distinct shapes, and are primarily responsible for detecting light. They send signals to the brain through retinal ganglion cells (RGCs) so the brain can form our perception of images. But our eyes do more than help create images of our surroundings. Our pupils respond to light intensity, constricting in bright light to reduce the amount of light entering the eye, and dilating when it’s darker to let more light in. Situated behind the pupil is a colourless, transparent structure called the crystalline lens. Ciliary muscles surround this lens. The muscles hold the lens in place, but they also play an important role in vision.

When the muscles relax, they pull on and flatten the lens, allowing the eye to see objects that are far away. To see closer objects clearly, the ciliary muscle must contract in order to thicken the lens.

Put like that, the function of the lens doesn’t sound that exciting, but consider it like this: Every time a young eye (under 40 yrs) looks at a different distance, from one object to another, everything remains in focus. Every time you look at a different distance the lens has changed shape a lightning fast speed by exactly the right amount of change in its curvature to keep everything in focus. You can’t catch it out because it changes so incredibly fast. An amazing piece of equipment.

Visual optics is an exact science. Any optical lens will focus light at a particular distance, which is determined by the curvature of the lens, as well as the refractive index (bending power) of the lens material. The crystalline lens in your eye, acts like thousands of lenses in one, because it can change its curves so quickly and accurately.

While you are young, you will be able to see at any distance with the same pair of specs regardless of your type of refractive error, because your crystalline lens is still very elastic. However, once you reach your forties, the crystalline lens loses its elasticity and can’t focus as quickly anymore (presbyopia). This is a normal aging process like grey hair and wrinkles, and will get progressively worse as you get older.

Now, back to my opening statement; One pair of specs cannot fulfil every visual function. We can now understand why this is so. Since the lens has lost its elasticity, it can only maintain a focus further away.

Reading glasses

Reading glasses can be made up to focus at 30 cms or 40 cms or 60 cms, but only one distance at a time. The optical lens has one constant curve which will focus at one point. However, in everyday life we need to continuously look at things at different distances. Your cell phone, computer screen or computer keyboard are all generally at different distances.

Multifocal Spectacles

These continuous focus lenses give a much better range of clear vision within a one meter distance and of course at a far distance. They do go some way in replacing the crystalline lens, but not all the way. For instance, the bottom strongest part of the multifocal is usually set at 40 cms. If you are a goldsmith working for long spells at 30 cms, they are not ideal. Moreover, looking at the computer screen with multifocals, your line of sight is likely to be over the stronger reading section, again not ideal. This is usually compensated for by tilting the head backwards, creating all sorts of muscular fatigue.

Horses for courses

Most patients over forty go to the optometrist with the expectation that one pair of magic specs will fulfil all visual functions. Just like it was when the crystalline lenses were still working their magic. What is required is an understanding of the limitations imposed on the visual system by presbyopia and that the solutions lie with more than one pair of glasses.

Setting up specs for an executive in an office environment, will not fulfil his visual requirement on the golf course. Specs for driving are not going to be perfect for vision on the canvass while painting. An artisan working on a lathe needs good vision at the right distance as well as visual safety. Here are some of the activities that require specially designed visual corrections:

• Cycling

• Golf

• Scuba diving

• Fitter and turner

• Musician

• Lawn bowls

• Jogging

• Hunting

• Architect

• Goldsmith

• Fishing

Apart from deciding on the best lens power for an person, there are many other considerations in terms of choice of lens material, frame design, lens coatings, safety features, lens design and multifocal type. The point is, each activity or visual demand, will require a different design of visual correction.

Go prepared

A visit to the optometrist should include an in depth discussion of what your work and recreational visual requirements are. Make a list of all the activities that are important in your daily routine or the activities that you are passionate about. Make a note of any unusual working distances these activities demand, and share all of this with your optometrist.

Vision is so important in order to deliver your best performance in work and play, and yet many struggle to come to terms with the fact that you need more that one pair of specs to fulfil all visual functions. It makes little sense to own thirty pairs of shoes, a Ferrari or a R100 000 mountain bike, but only one pair of specs.