The human eye is designed much like a camera. The retina is the camera film at the back of the eye that captures the image. The role of the human cornea and crystalline lens is to focus the incoming light onto the retina. If the light is not focussed correctly, then the person will have blurry vision and they will require glasses or a contact lens in order to see clearly.
There are 4 main types of focus error.
Myopia (near-sightedness). The eye can see well up close without glasses, but distance vision is blurry. Without glasses, the image is focused in front of the retina.
Hyperopia (far-sightedness). Distance vision is somewhat blurry, and vision becomes increasingly blurry as the object moves closer. Without glasses, the image is focused behind the retina. Children and young adults can compensate for far sightedness by “accommodating”. Accommodation involves the contraction of tiny muscles inside of the eye to increase the focusing power of the lens. Hyperopic patients who accommodate all day (instead of wearing glasses) are more likely to experience eye strain.
Astigmatism. The image seen is stretched in one direction. Most patients with astigmatism do not perceive that the image they see is stretched, but instead they simply report that their vision is blurry.
Presbyopia. Loss of accommodation due to stiffening of the crystalline lens with age. Presbyopia is the reason why individuals who have never required glasses before will begin to need reading glasses at around age 45.