The long-sighted person's eye is too short. Due to this beams of light are focused only behind the retina. At the retina a blurry image is created.
For a long time long-sightedness can be compensated by the eye lens. By deforming the lens increases the refraction of the eye whereby the beams of light are projected onto the retina. In most cases long-sighted people - up until old age - are very well capable of making out things at a distance.
However, long-sighted persons' reading abilities cease earlier. The deforming ability of the lens is already required looking in the distance. This leaves only little room left for adapting for reading. With increasing age the deforming ability of the lens decreases; the long-sighted person lacks reading glasses earlier.
Long-sightedness as well can be corrected by the means of glasses, contact lenses or surgery (including laser treatment).