The human eye refracts light emitted off objects and onto the retina to bring the objects being viewed into focus. This involves many functions and processes.
Light rays pass through the eye’s clear front cover called the cornea.
Light passes through the eyes pupil, the eye’s window to the world. The pupil is surrounded by a sphincter call the iris; the eye’s colored ring.
Light passes through the eye’s crystalline lens, which constricts to help light rays come to focus at one focal point.
Light rays travel to and come to rest on the retina resulting in clear vision.
Once light rays come to rest on the retina a signal is carried done the optic nerve to the brain.
The brain receives the signals and interprets them into a picture(s).
The brain tells the eyes what it sees by forming the picture.
Typically, vision is measured by the Snellen Visual Acuity chart, which is where the measurement of 20/20 comes from. 20/20 means the normal eye of a person can see clearly at a distance of 20 feet away.
A person with 20/40 vision at 20 feet away from an object will see what an eye sees when it is 40 feet away from the object.
A person with 20/15 vision at 20 feet away from an object will see what an eye sees when it is 15 feet away from the object. This means this person will see details more clearly than someone with 20/20 vision.
There are many reasons a person may not have 20/20 vision; some of which are not correctable such as amblyopia or better known as lazy eye, trauma to the eye, or even diseases such as diabetes or macular degeneration.
On the other hand there are causes of less than perfect vision which are correctable; these are known as refractive errors. “Refractive” refers to the bending of light rays.
When light focuses accurately onto the retina, meaning when light rays come to rest on one focal point clear vision is achieved. The result is often 20/20 vision. However, there are millions who cannot bring objects into focus clearly without a crutch such as (prescription eyeglasses) or contact lenses. This is because they have refractive errors. There are three main refractive errors; they are:
Myopia – also known as nearsightedness is an eye that sees objects better close up rather than at a distance. This is due to light rays being bent too much or too soon and focuses hypothetically in front of the retina.
Hyperopia – also known as farsightedness is an eye that sees objects better at distance rather than close up. (Do not confuse with Presbyopia.) Hyperopia occurs due to light not being bent enough or too late and hypothetically comes to focus past the retina.
Astigmatism – a condition due to an irregular shaped eyeball and causes light rays to bend erratically resulting in blurred vision at any distance.
Presbyopia – a condition which means a person has reached the age where the crystalline lens has lost elasticity and can no longer focus on objects up close. It is also common a person could be both myopic and hyperopic too requiring reading (eyeglasses) and distance (eyeglasses).