Although vision impairment can affect people in many different ways, there are generally four broad categories of sight loss

Central field loss
Central field loss occurs when a person loses the ability to see the visual field directly in front of them, whilst the peripheral area is still visible.
As much of the processing associated with discerning spatial detail occurs in the central field, this impairment can also lead to problems with depth perception as well as fine colour discrimination. As a result, reading is almost always affected.
People with this impairment will still be aware of their general environment but will have problems with the fine detail within it.

Peripheral field loss
This is almost the exact opposite of central field loss: people with this impairment can see straight ahead, but lose the details of the periphery of their vision. As such, people with peripheral field loss are almost looking at the world through a tube.
Peripheral field loss can influence mobility, as people find it harder to avoid hazards and obstacles. Awareness of people and things approaching from the side can also be lost. Because of the nature of this impairment, sufferers can still perform well on traditional visual acuity and colour vision tests.

Combined central and peripheral field loss
This impairment can lead to people suffering from both central and peripheral field loss of sight.

Reduced contrast
Reduced contrast causes a loss of contrast, resulting in a washed out effect. Strong light sources can wash out the whole environment, and this condition can be caused by cataracts.