Understanding myopia: differences between normal vision, myopia, hyperopia and astigmatism

Although conditions like myopia, hyperopia, astigmatism, and other vision impairments are well-known, surprisingly few people fully understand how they work. In this section, we will provide a clear explanation of these different conditions.

Normal vision

In normal vision, when you look at an object, the image is focused directly on the retina, allowing you to see clearly both at a distance and up close without any need for adjustments.

Myopia

Myopia, or nearsightedness, is a refractive error where distant objects appear blurry because the focus point is in front of the retina. However, close-up objects can be seen clearly without issue.

Hyperopia

Conversely, hyperopia, or farsightedness, occurs when the focal point is behind the retina. This makes it difficult to see clearly both near and far without adjusting focus. The eye continuously strains to focus, which often leads to eye fatigue.

Astigmatism
Astigmatism is a condition in which the position of the focal point varies depending on the direction of light coming into the eye, mainly due to distortion of the cornea and lens, causing blurred or distorted vision. Often, astigmatism coexists with myopia or hyperopia, and is referred to as myopic or hyperopic astigmatism, depending on the underlying refractive condition.

How to correct vision

Myopia, hyperopia, and astigmatism can all be corrected by altering how light is refracted, ensuring that light entering the eye is focused on the retina.

Common correction methods include contact lenses, glasses, LASIK surgery, and orthokeratology, especially for myopia and myopic astigmatism.

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Types of myopia: axial and refractive

Myopia can be classified into two main types: axial myopia and refractive myopia. The distinction is based on the underlying cause of the refractive error.

What is axial myopia?

Axial myopia occurs when the length of the eye’s axial length (distance from the cornea to the retina) is longer than normal. In this case, light does not focus on the retina when looking at distant objects, and the focus point falls in front of the retina. 

Even a slight increase in the axial length (as little as 0.5 mm) can cause blurry vision. This condition typically does not improve with training or eye drops. Axial myopia often progresses during growth as ocular axis lengthens, and in some cases, it may continue to worsen even after adulthood.

What is refractive myopia?

Refractive myopia occurs due to an excessively strong refractive power of the cornea and lens. This type of myopia can develop when the lens becomes strained from prolonged close-up activities, such as reading or using a computer.

The lens expands to focus on near objects, but if this strain is prolonged, the lens remains distended, making it difficult to focus on distant objects. Refractive myopia can sometimes be improved through eye relaxation techniques, such as eye exercises or eye drops. However, this type is less common than axial myopia.

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Normal vision: when the ocular axis length (eye depth) is ideal

Normal vision occurs when light entering the eye is properly focused on the retina. When the ocular axis length is ideal, the lens bulges to focus on distant objects, ensuring that the image is clearly projected onto the retina. This state is referred to as “normal vision.”

When the ocular axis length is too long

Myopia is a condition in which distant objects appear blurry because the light is focused in front of the retina.

It is a type of refractive error that occurs when the distance from the cornea to the retina (ocular axis length) is too long, or when the refractive power of the cornea and lens is too strong.