These disorders affect the retina, which is the light-sensitive tissue in the back of the eye, in which the first stages of seeing take place. In RP, sight loss is gradual but progressive.
Some people retain a small area of useful vision well into old age.
The retina in your eye serves a similar purpose to a film inside a camera. Light is focused by a lens at the front of the camera onto a light-sensitive film at the back, to form a picture. In a similar way, light entering your eye is focused onto a light-sensitive tissue which lines the inside of the eye at the back. This tissue is the retina. The retina consists of two main layers, a thin one called the pigment epithelium and a thicker one, made up of many layers of cells, called the neural retina.