Myopia, hyperopia, short sightedness, long sightedness, near sightedness, far sightedness… While some or all (or perhaps none) of these terms may sound familiar to you, there is often some confusion about what short sightedness and long sightedness actually mean. Many people who are short or long sighted themselves in fact often don’t even know which one they are. Here we’ll try to explain what these terms mean, as well as the difference between long and short sightedness.
How Refractive Error Works?
Both long sightedness and short sightedness are types of refractive error. Refractive error occurs when the power of the eye, which we commonly call its prescription, is mismatched to the length of the eyeball. This results in light focusing at a point anywhere other than the retina at the back of the eyeball. Both long sightedness (also known as hyperopia or far sightedness) and short sightedness (also called myopia or near sightedness) may be found in conjunction with another type of refractive error called astigmatism.
For perfectly crisp vision, we want light passing through the eye to be refracted, or bent, to a degree such that all rays come to a sharp focal point right on the retina. This relies on several factors:
- The cornea. The cornea and the layer of tears covering it are the first refractive surface that light must pass through as it enters the eye. The cornea is the clear dome of tissue at the very front of the eye. The cornea with the tear film accounts for approximately two-thirds of the focusing power of the eye.
- The lens. The crystalline lens sits behind the coloured iris, held in place by a ring of flexible fibres. This lens contributes the remaining one-third of the eye’s refractive power when in a relaxed state. However, the lens is also able to adjust its shape in order to focus on nearer objects. This process is called accommodation. Our accommodative ability gradually diminishes with age with this decline eventually becoming noticeable around 45 years of age. This marks the manifestation of a normal age-related process called presbyopia.
- Axial length. This is simply the medical term referring to the length of the eyeball. The average axial length of an eye without refractive error, known as emmetropia, is 23.5mm.
The clearest difference between long and short sightedness is the axial length. From an anatomical perspective, a long sighted (hyperopic) eyeball’s axial length is too short for its refractive power, resulting in light focusing behind the retina. In a short sighted (myopic) eyeball, the axial length is too long for its refractive power. This means light focuses too early, in front of the retina.
Astigmatism is slightly different in that it is often a product of an uneven curvature of either the cornea or the crystalline lens. This results in light being focused at not one but two different focal points within the eye. One or neither of these focal points may fall onto the retina, leading to blurry vision.
However, if you have either long sightedness or short sightedness, you would have realised that your vision, in some situations, is not too bad. This is where things can get a little complicated.
Experiencing The Difference Between Short and Long Sightedness
It is not an uncommon misconception to believe that people with long sightedness cannot see up close and those with short sightedness simply cannot see far away. In reality, it’s much more complex than that.
In the case of short sightedness, myopic people will have blur for long distance vision. Often, they will have quite reasonable near vision, such as for reading, but this depends on their degree of short sightedness. Very short sighted people will still require glasses for reading or other near work, or else will need to hold an object uncomfortably close in order to see it clearly. Oftentimes people with myopia will wear glasses for long distance vision but prefer to remove their glasses for reading. This is more apparent for people with presbyopia. Young people with short sightedness may not find any difference with their near vision with or without their glasses as their accommodative system is flexible enough to compensate for the change in focus.
When it comes to long sightedness, age plays a big part in what a hyperopic person can see. Mentioned earlier was the role of the crystalline lens and how presbyopia begins to manifest around the mid-40s. In a pre-presbyopic person with long sightedness, an active accommodative system is often able to compensate for the eyeball’s shorter axial length. However, as we get older and our accommodative ability declines, doing this becomes more and more difficult. Because near vision requires the most accommodation, this is the first to deteriorate. In many with long sightedness, difficulties with reading vision may become apparent before the typical mid-40s manifestation of presbyopia. In addition to difficulties focusing on close objects, you may also experience eyestrain or headaches. Eventually, long distance vision will also become blurry and strained as the accommodative system is no longer flexible enough to bring things into focus.
Hyperopia, myopia, astigmatism, and even presbyopia can typically be easily corrected with glasses or contact lenses. Many patients with these refractive errors will also find themselves to be suitable candidates for refractive surgery, whether laser eye surgery or another technique.
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