It is not meant to be a comprehensive discussion of either, but does provide an introduction. A more scientific paper on accommodation and presbyopia can be found here and is also linked through my research web pages. So when the normal eye is relaxed, distant objects are in focus. A short sighted or myopic eye is focused at a point nearer than optical infinity when relaxed and must be corrected with spectacle or contact lenses to again be focused at optical infinity. Accommodation occurs in young people when they look at a near object, such as when reading.
Accommodation is an optical change in the eye.
It occurs due to an increase in the optical power of the crystalline lens inside the eye. When the muscles inside the young eye contract, the muscles allow the optical power of the lens to be increased so that near objects can be brought into focus on the retina. This process occurs continuously, but gradually from soon after birth. The progression of presbyopia generally goes unnoticed until accommodative amplitude is almost completely lost as one nears age 45-50. It is only at this age that the accommodative amplitude is reduced sufficiently to cause a problem for typical near vision tasks such as reading.