The Difference Between Long and Short Sightedness.

If you’ve ever assumed long-sightedness is the ability to see far away but not up close and short-sightedness is just the opposite, you wouldn’t be the only one. However, the difference between long- and short-sightedness is not as straightforward as one experience being the opposite of the other.

 

A LITTLE BIT OF EYE ANATOMY

There are several anatomical components that are involved in the creation of vision, and this isn’t even going into the visual processing and interpretation  that takes place in various parts of the brain. From an anatomical perspective, to achieve sharp vision light must pass through:

• The tear film at the interface between the surface of the eyeball and the air
• The cornea, which is the transparent dome of tissue at the front of the eye
• The fluid between the cornea and the crystalline lens behind the iris (the coloured part of the eye); this fluid is called aqueous humour
• The crystalline lens; this lens is suspended behind the iris with fibres known as zonular fibres, and are attached to a ring of ciliary muscle. This lens is able to change its shape and thereby change the focus of light by tensing or relaxing the ciliary muscles in a process called accommodation
• The gel between the lens and the retina at the back of the eye, called the visual processing and interpretation.

 

After passing through all these optically transparent media, light rays must then focus to a point right on the retina in order for us to perceive our sight as clear and sharp.

In a perfect eye, the degree of refraction (bending) of light through all the components mentioned above is matched to the physical length of the eyeball such that without any effort, when the eye is fully relaxed and looking into the distance, light will naturally focus to a sharp point right on the retina.

Refractive errors – such as long-sightedness and short-sightedness – arise when these factors do not match up.

 

THE DIFFERENCE BETWEEN LONG AND SHORT SIGHTEDNESS

Short-sightedness, also known as myopia, occurs when the length of the eyeball is too long for the refractive power of the eye. People with short-sightedness will tend to find that their near vision, such as for reading, is reasonably clear with no spectacles or contact lenses while optical correction is required for clear distance vision. This holds true only for some people with increasing age, depending on the degree of their short-sightedness.

At some point, a person with a low degree of short-sightedness will find they still require a reading prescription as they get older; this occurs if a person’s normal age-related decline in near vision (presbyopia) exceeds their natural ability to see up close. Patients with moderate degrees of myopia may find they can retain clear reading vision by simply removing their distance glasses.

Long-sightedness, or hyperopia, works a little differently, but can be considered opposite to myopia in the way that long-sightedness occurs when the length of the eyeball is too short for the power of the eye. You may have noticed that out of the refractive components listed above, only one could be controlled – the power of the lens during the process of accommodation. When we are young, the lens is flexible and accommodation is quite active.

Young patients with long-sightedness therefore often find that their sight remains clear and comfortable at both distance and near, depending on the magnitude of their prescription. However, it means that even to view distance objects the eye is exerting accommodative effort to clear the vision; this effort increases when viewing objects as they come closer.

As we get older, the lens naturally becomes stiffer and slowly loses its accommodative ability, which is why many people notice difficulty with their reading vision around their mid-40s as presbyopia sets in. Because patients with long-sightedness are already relying on accommodation to see at a distance and even more so at near, depending on the degree of long-sightedness, they will eventually find that both their far and near vision will deteriorate with age and they may require spectacles for both activities, likely starting with near vision first.