Adapting

"Little Piggy" wear his new spectacles - image ©ComedyCentral

Adapting to new Spectacles

Sometimes a new spectacle correction can take quite some getting used to.

When you are being tested the main consideration is which lens gives the sharpest vision, however (rarely) if there is a significant change in the refractive error, or if someone is particularly sensitive to a change, they may have difficulty adapting to the new prescription- this does not indicate that the glasses are wrong- as long as the “straight ahead” vision is clear, that is unlikely.

In the vast majority of cases if the patient perseveres, most effects will diminish or completely resolve within a week to ten days. Although they do try, some patients find adaptation difficult- some of these effects can be unpleasant. In those cases a modified correction, which may not be as clear may be necessary.

There follows an explanation of some of these adaptation effects.

Size and distance judgment

Plus lenses, used to correct long sight, have a side effect of making the image on the back of the eye (the retina) larger. Minus lenses, for shortsighted correction, make the retinal image smaller. This phenomenon is an undesirable but unavoidable fact of correction with spectacle lenses.

Plus lenses therefore make objects look bigger, but in the example of our feet, which the brain knows don’t change size, if they look bigger, the brain wrongly decides “they must be closer”. So we feel shorter. Hands look bigger so arms feel shorter. Everything else looks bigger, which adds to the sensation that we have shrunk!

The exact opposite is true for myopic (shortsighted) minus lenses, but because in general myopia requires correction at a younger age, it is less likely to cause problems- youngsters seem to adapt quicker, and their correction usually gradually increases over time, which eases them into wear.

Difficulty is most likely in adult first time spectacle wearers, who usually require a plus prescription, but people who have been wearing contact lenses, and never spectacles may suffer the same symptoms if they have to stop contact lens wear.

These effects will lessen over time, but they can be disturbing initially.

Giddyness

Another rare complaint is that of spectacles making the wearer feel sick or giddy, which is again caused by the magnification. Again we will discuss plus lenses, but the opposite is true for minus.

Various brain reflexes control our eyes, and some of the most robust are the Vestibular, which are controlled primarily by the ears. Regardless whether the eyes are open or closed, if we want to, we can keep our eyes fixed on the same object, even if we turn and tilt our head. If this reflex tells the brain the head is tilted 20° forwards, the eyes will be tilted 20° upwards to compensate, same for left and right movements.

But! if the image is now enlarged by a plus lens, a 20° upward movement would not be enough- if the object looks bigger the eyes would need to make a larger angular deviation to remain “on track”, perhaps 22°. This can cause the same effect as motion/seasickness, which is basically caused by a disagreement between what the eyes and the ears tell the brain.

We can explain the cause, but it may just be a case of trying to get used to it.

Sloping

Astigmatism, as previously explained elsewhere, is due to an uneven curvature on the cornea, or due to the lens inside the eye being tilted. This requires a lens which has a different strength one way as the other-maximum and minimum strengths will always be at 90° to each other. Talking plus lenses again, that means that one meridian is more plus than the other, which results in more magnification one way than the other.

Imagine looking at a circle- if it is magnified more one way than the other, the retinal image will be not a circle, but an oval. In another common sense judgment the brain gets it wrong- how can an object known to be round look oval? It would if it was tilted… imagine looking down on a coffee cup- circle. Look at the cup at an angle and we would see an oval. So the brain can introduce a tilt to things which can’t be true but feels so. Desks and floors may slope and door frames may appear distorted.

Again these strange perceptions should lessen in time, but can be disturbing.

Rarely it is necessary to adjust the strength of the spectacle lenses to minimise these unpleasant effects- with a modified correction, the wearer may feel more comfortable, although they would agree they don’t see quite as clearly as with the previous lenses.

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