There are a variety of refractive errors, including myopia (short-sightedness), astigmatism and stigmatism.
With myopia, rays of light from a distant object are focused in front of the retina, either because the eye is too long in axial length from the front to back, or the cornea is too steeply curved with too much focusing power. Sometimes, myopia is a combination of both these factors.
A negative or minus spectacle or contact lens is required to correct short-sightedness which reduces excess positive focusing power. When a short-sighted person isn’t wearing glasses, their distance vision will be blurred. The greater the amount of myopia, the more blurred their view of the world. Even a relatively modest amount of myopia will cause a considerable blurring of distant objects.
Hyperopia (Hypermetropia, long-sightedness)
Hyperopia is slightly more complex than with myopia. With this refractive error, rays of light from a distant object are brought to focus behind the eye because either the eye is either too short in axial length from the front to back, or the cornea is relatively flat or insufficiently curved with insufficient focusing power. Correction happens with positive or plus lenses.
The complicating factor of hyperopia is that younger patients can supply some or all of the additional power required when their crystalline lens inside the eye changes shape to compensate for the refractive error. This natural corrective ability declines with age and, ultimately, all hyperopic patients will require optical correction for both distance as well as reading.
In myopia and hyperopia, the image of a distant object is brought to a single point focus in front of or behind the retina respectively. In the case of astigmatism, the cornea is more steeply curved in one direction (principal meridian) than the other direction.
When the cornea has an elliptical or oval shape, this refractive error is astigmatism. Instead of a point focus, two line foci are produced. The spectacle lens required to correct this is termed an astigmatic, cylindrical, toroidal or toric lens. Oftentimes, astigmatism co-exists with either myopia or hyperopia.
Presbyopia (the need for reading glasses)
Almost everyone will require reading glasses at some time as the ability of the eye lens to focus on close objects declines with age. This is termed presbyopia and is completely normal as you age – even for those who used to have perfect sight (Emmetropia).
Those who wear glasses for myopia or hyperopia will require an additional reading prescription, typically a bifocal or multi-focal (progressive, varilux, varifocal). Depending on the amount of myopia, some will be able to read small print easily by removing their glasses.