Two key terms to know

Refractive Error The condition which causes the image projected onto the back of the eye to be out of focus. This causes blurry vision, either at distance or close up.

Self-refraction The process where a user makes their own glasses, by adjusting the lenses until they can see clearly. By then 'setting and sealing' the glasses, they turn the adjustable device into a normal wearable pair of glasses.

Other useful terms

Screening
The simple process where a healthcare worker (who requires minimal training) determines if a patient requires glasses, using some quick and simple tests.

Subjective refraction
The traditional method of finding out what lenses a patient needs, where an optometrist goes through a procedure to determine when you are seeing optimally. This is the method used in most of the developed world.

Myopia (short-sightedness)
Rays of light from a distant object are focussed in front of the back of the eye. Myopia occurs either because the eye is too long or the the cornea and lens are too powerful to produce a clear image at the back of the eye.

Hyperopia (long-sightedness)
Rays of light from a distant object are focussed behind the back of the eye, either because the eye is too short or because the lens and cornea of the eye are not powerful enough to bring an image into focus at the back of the eye.

Astigmatism
Astigmatism occurs when the curvature of any given element of the eye differs across its surface (i.e. the lens in the eye is shaped more like a football than a soccer ball). This lack in uniformity across the surfaces of the eye's lens or cornea prevents it from producing a sharply focused image at the retina. Astigmatism usually produces an image that is more defocused in one direction than another.

Presbyopia
A loss of ability to change the focussing power of the lens due to hardening of the lens tissues, leading to a lack of ability to focus on close objects, such as when reading. Everyone will experience presbyopia as they age, and unless they are also short-sighted to the correct degree, will require reading glasses to see close objects. Reading glasses are the easiest way to help prevent the curtailment of productivity caused by a lack of close vision, helping people in the developing world obtain income for many years longer.

Visual acuity
The resolution with which a person can see. Someone with excellent visual acuity can see small objects in the far distance, and someone with poorer visual acuity can only see larger objects at the same distance.