Unfortunately, no, long sightedness is not exactly as simple as being the opposite of short sightedness. People with long sightedness can in fact often still see up close, and those with short sightedness are often quite content with their long-distance vision even without glasses or contact lenses. The terminology can be a little misleading as the difference between long and short sightedness is not quite as straightforward as it may sound.
Let’s differentiate first these two refractive errors and later discuss what’s the right eye treatment for them.
Refractive Error and Anatomy
Vision is dependent on multiple factors, some of these simply anatomical and others cognitive. From an anatomical point of view, the refractive components of the eye – that is, those parts of the eyeball involved in the bending (refraction) of light – include:
- The tear film – the thin layer of fluid at the very front surface of the eye. In addition to providing protection and lubrication for the eye, it is also the first surface that light passes through to form vision.
- The cornea – the clear dome of tissue that sits over the coloured iris. The cornea is designed to be optically transparent to allow full transmission of light; its curvature is also largely responsible for how light is refracted.
- The crystalline lens – this sits just behind the coloured iris and is held in place by the ciliary muscle and zonular fibres. This arrangement allows the lens to flex its shape to adjust the focus of light in a process called accommodation. We accommodate when we want to bring the focal point of light forward within the eye, such as when we are looking at something up close.
The term “refractive error” covers both long sightedness, short sightedness, as well as astigmatism, and occurs when there is a mismatch between the refractive power based on the eyeball’s refractive components and the eyeball length, known as axial length.
The Difference Between Long and Short Sightedness
Long sightedness goes by a few other names – far-sightedness, hyperopia, or hypermetropia. Although it may seem most intuitive to simply think of long sightedness as the ability to see at long-distance but not at close, most younger patients are actually quite clear and comfortable at all viewing distances.
Long sightedness is a situation where the axial length of the eyeball is too short for its refractive power; that is, when the eye is relaxed, light is refracted to focus at a point behind where we want it to be for clear vision, which is the retina at the back of the eye.
However, a patient with an active accommodation system and long sightedness is able to control the focus of their crystalline lens in order to bring forward the focal point of light entering the eye, landing this point right on the retina for clear vision. This does mean that even for long distance viewing, a long sighted patient is always exerting some accommodative effort to clear their vision; this effort is increased when viewing closer objects.
To add to the complexity, around the mid-40s a natural age-related phenomenon sets in – presbyopia. This refers to the progressive inflexibility of the lens inside the eye, and eventually accommodative ability is lost entirely. The combination of the age of the patient and the degree of the far-sighted prescription therefore is what really determines how clearly someone can see at a certain distance.
For example, a 20-year-old patient with a low long sighted script will be quite happy at both short and far viewing distances while a 20-year-old with moderate long sightedness may be fine for long-distance vision but struggle with eyestrain and headaches during reading; a young patient with very high hyperopia can have blur at both distance and near vision. Alternatively, a presbyopic 50-year-old patient with low hyperopia may see quite clearly for far-distance while still having difficulty reading up close.
Luckily, short sightedness is much easier to explain. Also known as near-sightedness or myopia, short sightedness occurs when the axial length of the eyeball is too long for its refractive power; that is, when the eye is gazing into the far distance, light will focus to a point in front of the retina. However, as an object approaches closer, the nature of optics means the image created by this object moves further back into the eye and may eventually focus onto the retina – this explains why people with short sightedness can often see well up close but require spectacles to clear their far vision.
Presbyopic patients with short sightedness will find at some point that they prefer to remove their distance spectacles in order to see clearly for reading up close; an alternative to this is the use of multifocal glasses that contain both distance and near vision sections within the lens. Depending on the degree of short sightedness, some patients may be happy without any vision correction, such as a young person with a low degree of short sightedness who may feel they see well enough at distance without correction, and are also able to accommodate to see clearly for reading.
Although understanding the difference between long and short sightedness may not have been as simple as you expected, both types of refractive error can be addressed with various strategies including glasses, contact lenses, or refractive surgery.
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