How The Eye Sees

Refractive error occurs when cornea and lens are unable to focus light.

The eye functions like a camera. Light rays enter the eye through the cornea (the clear window in front), pass through the pupil (the hole in the centre of the iris), and are then focused through the lens, finally reaching the retina at the back of the eye. When light rays land on the retina, they form an upside-down image. The retina converts the image into impulses that travel through the optic nerve to the brain, which converts them into upright visual images.

Refraction and Refractive Errors

Vision is clear only if the cornea and lens correctly bend or “refract” the light rays and focus them on the retina. Blurry vision may be due to “refractive error” - the failure of the cornea and lens to focus light properly. Prescription eyeglasses, contact lenses, and refractive surgery correct or improve refractive errors by focusing light rays closer to or directly onto the retina.

Related: Healthy Eyes, Clear Vision

Refractive Errors Explained

Unlike myopia, hyperopia is caused when the cornea of their eye is not curved enough.

How the Normal Eye Works

In order to see clearly, objects need to sharply focus at a point on the retina known as the fovea. The retina is analogous to the film of a camera. Light is brought into focus by the cornea and lens of the eye. The cornea’s focusing power is matched to the eye’s length in the normal eye.


For nearsighted people, the cornea is overly curved (focusing power too high), or the eyeball is too long. This situation brings images of distant objects (e.g. street signs) to be focused in front of the retina. A blurred image falls on the retina as the light rays spread apart after the focal point.


For the farsighted, the cornea of their eye is not curved enough (focusing power too low), or the eyeball is too short. This combination of factors results in the focal point to fall behind the retina. A blurred image is seen as light rays have not yet been brought into focus when they reach the retina.


If you have astigmatism, the cornea of your eye has a non-spherical shape (like a rugby ball or the back of a spoon) and does not bring the light rays to focus at a single point. Instead, it focuses images over a spread of points producing a blurred image. Astigmatism often occurs in combination with nearsightedness or farsightedness


As the normal eye undergoes ageing, the lens loses its ability to change its shape. Focusing on near tasks, including reading, will then require the help of reading glasses. This usually begins to affect most people from the age of 40 onwards.

Presbyopia can be treated with either LASIK or PRK. One eye can be corrected to see far, and the other eye corrected to see near. This is also known as monovision. By doing this, the person can then see both far and near without the need for glasses or contact lenses. Confusion does not occur, and the brain easily adapts to this new vision after 1 to 2 weeks. There is also no problem with activities such as driving or playing golf. The blending of vision between the two eyes can be further enhanced using aspherical treatment to increase the depth of focus of both eyes.

Related: Look Into It

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