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20:20 vision is a popular term used to describe well-functioning eyesight. Ideally, light should converge at a single point on the retina, found at the back of the eye. But what happens when it doesn’t? You’ll probably find yourself at the optometrist... ideally Specsavers, of course!

We’re going to look at the most common visual defects we see in our customers. We’ll introduce you to how each of these visual defects are corrected, and what that might look like on a prescription. Scroll down to learn more.

Pictured Below: A cross-section of an eye functioning without defects.
The light is converging on a single point on the retina.

MYOPIA

or 'SHORT SIGHTEDNESS'

If you are Myopic, or ‘shortsighted’, as a general rule you’ll have difficulty seeing distant objects clearly, but are able to see well for close-up tasks.

Why? A myopic eye is too 'strong' or the eye is too long. This causes the light to converge in front of the retina.

Sam has myopia and is able to read the items on the menu but is having difficulty focusing on further objects in the scene. Drag the slider to see how the light fails to converge on the back of the retina as the focus distance increases.



Sam's Eye


Correcting Myopia

To correct myopia we use a concave or ‘minus’ lens, which diverges light (or, points it ‘away’) before it reaches the eye. This counteracts the effect of the eye’s magnification being too ‘strong’.

On a prescription, this is labelled as sphere or ‘SPH’. The strength of correction is measured in a unit of measurement called dioptres.

Sph Cyl Axis Add
-1.00 - - -

Let’s look at that same scene, this time with Sam trying to focus solely on the trees in the distance. Using the slider, vary the corrective power of the minus lens to make the light converge on the retina.







HYPEROPIA

or 'long sightedness'

Hyperopia is the opposite of myopia. If you’re long sighted, as a general rule you’ll have difficulty seeing close objects clearly, but are able to see well for distance.

Just as a myopic eye can be too ‘strong’ (or long), a hyperopic eye can be either too ‘weak’ or too short. This results in light converging behind the retina.

Sometimes people with hyperopia don’t notice any blurring up close because they can compensate, or accommodate, using the crystalline lens inside the eye. This can lead to eyestrain and fatigue if not corrected.

Julie has hyperopia and is able to see John mowing the lawn but has difficulty with objects at closer distances. Drag the slider to see how the light fails to converge on the back of the retina as the focus distance decreases.



Julie's Eye

Correcting Hyperopia

To correct hyperopia we use a convex or ‘plus’ lens - the opposite of how we correct myopia. A convex lens will converge light (or, point it ‘towards’) before it reaches the eye. This light will then be further converged by the cornea and lens to produce an image on the retina. The power of the correction is also measured in sphere, but this time in plus (+) dioptres.

Sph Cyl Axis Add
+1.00 - - -

Now let’s revisit the scene, this time Julie focussing solely on the magazine at reading distance. Using the slider, vary the corrective power of the minus lens to make the light converge on the retina.








astigmatism

The ‘rugby ball shaped’ cornea

Folks that have astigmatism may experience a general inability to focus on objects, near or far. They may also perceive objects to be slightly distorted.

Most commonly, astigmatism is caused by an irregularly shaped cornea. Instead of the cornea being perfectly symmetrically round, , like a tennis ball it is shaped more like a rugby ball or olive, with a steep curve in one direction and a flatter curve in the opposite direction.


Because the cornea is not symmetrically round, the ovoid shape causes the light to converge on multiple points, that could fall anywhere. The eye could be misshapen in any direction - or, in optical terms ‘meridian’.

Interact with the rotation of this meridien below to see how it can affect how the light fails to converge on a single point on the retina.


ROTATION OF MERIDIAN:

Correcting Astigmatism

Astigmatism is corrected using a toric lens. Toric lenses are made up of a cylinder orientated in a specific direction called the axis. The optometrist will find the correct strength of cylinder (cyl) measured diopters. They will also determine the correct orientation for the cyl which is then noted as the axis. Having the correct strength of cyl at the correct orientation or axis will create a clear, single image on the retina. Let’s examine each of these elements separately.

Sph Cyl Axis Add
- -1.00 180° -

Astigmatism: understanding cyl


Imagine you’re making scones. Your dough is rolled out evenly and flat. Now you take a circular cutter to get the round shape.

Now, take a rolling pin and press it down. It ‘cuts’ a circular section from the scone. The rolling pin affects the dough in roughly the same way cylinder does when it’s added to a lens.

The size of that rolling pin is like the size of the cylinder in a lens.

Astigmatism: Understanding Axis

Axis Axis describes the orientation of the cylinder in the lens. This will normally be in the opposite direction to the steepest curve on the cornea. The diagrams below show a red line along the steepest curve of the cornea, notice that the axis it set at 90 degrees to the steepest curve.

An eye with a steep curve at 180 degrees would need a corrective axis at 90 degrees.

An eye with a steep curve at 90 degrees would need a corrective axis at 180 degrees.

An eye with a steep curve at 45 degrees would need a corrective axis at 135 degrees.

presbyopia

"old sight"

Presbyopia is a condition where the flexible crystalline lens and the supporting muscle inside the eye become affected by age.

The lens is less able to change shape (accommodate) for focussing on near tasks, like reading. This usually starts to happen to people in their forties and they notice difficulty seeing small print.

It’s common to hear about presbyopes (without corrective lenses) trying to hold reading matter further and further away - until their arms are not long enough!

Experiment with changing the focal distance in this scene and pay special attention to the lens changing shape. See if you can spot the point at which the presbyopic eye fails to accommodate. Observe the effect on how the light converges and the resultant screen.




normal eye

presbyopic eye

Correcting Presbyopia

To correct the vision of a presbyopic eye a simple magnifying lens is introduced, its power is measured with ‘add’.

Often people with presbyopia may only have a section of their corrective lens dedicated to their ‘reading add’. This avoids the need to swap, or constantly remove spectacles. These lenses are known as multifocals.

Sph Cyl Axis Add
- - - +1.00

Let’s check out this scene again, the focal point fixed on the guidebook. Vary the power of magnification to assist the crystalline lens to accommodate.






presbyopic eye

We’ve now covered the basics. These are some of the most common visual defects that we see in customers visiting our stores.

Remember – throughout this site we’ve simplified things by separating each of the visual defects. Customers will commonly need to have a combination of these corrections prescribed by the optometrist to help achieve clear vision. Look out for prescriptions containing Sph, Cyl, Axis and Add elements in store.