Blindness or visual impairment may result from:

• disease
• defective function of parts of the eye
• defects in the shape of the eye
• congenital defects, or those present from birth
• irritation
• injury
• accidents

The leading causes of blindness, in order of incidence, are:

• cataracts
• glaucoma
• diabetes
• vascular disease
• trauma
• heredity

People with visual impairments have a wide range of abilities and limitations. People who are described as legally blind may be able to read large print and move about without mobility aids in many or all situations. They may be able to perceive light and darkness, and perhaps some colors. They may also not have any of these skills.

There are many kinds of visual impairments. Some people with visual impairment may have better vision on some days, and worse vision on other days. This can depend on factors like their environment, fatigue, and lighting. For example, too much light bouncing off light-coloured walls and floors can result in glare. Glare can make it difficult or uncomfortable to travel a long corridor or around a room. Low light levels can result in heavy shadows. This makes it difficult to perceive hazards like stairs or changes in floor surfaces.

Some people with congenital visual impairment (since their birth) may have learned to read Braille and use tactile orientation guides. People who have lost their sight later in life usually have visual memories of colour and scale, and concepts such as reflections that those who have been blind since birth do not have.

According to the Department of Social Welfare, out of 98,452 voluntary registrations in 2000, 13.743 people or 13.96 percent of them were visually impaired. This compares to approximately 108,000 people as of June 2002 but no specific breakdown by type of disability is available.