The human eye is a complex optical system. The lenses of the human eye consist of the cornea and the lens. Each lens has a focal length, which is the distance at which light rays from an object at infinity are refracted to form a defined image. This value is constant. A healthy eye has a focal length of 23.5-24 mm. This is the distance at which the retina is located. Such eyes see well.
Information about an object is transmitted from the retina to the brain via the optic nerve, where it is analysed. In a healthy eye, the image is clearly focused on the retina and visual acuity is 100%.
Visual impairments
One of the most common vision problems is nearsightedness (Latin “myopia”) a condition in which objects appear in focus in front of the retina. It can be caused by a longer shape of the eyeball or a greater refractive power of the eye lenses (cornea and lens). It also leads to a shortening of the focal length. Distant objects are blurred.
Hypermetropia (farsightedness) is a condition in which light is focused behind the retina, the focal length is less than 23.5-24 mm and the optical power of the cornea is low. Near objects are blurred.
Astigmatism is a problem when the cornea has two different points of refraction, i.e. different optical powers, perpendicular to each other, each with two focal lengths. This causes the image to be focused as a straight line rather than a point.
Presbyopia (age-related changes) occurs after the age of 40, when everyone’s body is actively changing. The transparency of the lens changes, the tissues become less elastic and the ability to focus the image is gradually lost. Usually such a person needs reading glasses or glasses for seeing at medium distances.
As people age, their bodies also change, and their vision becomes blurred when they look at distant objects. They may need to wear either distance glasses or reading glasses for near or intermediate distances.
Presbyopia and other visual impairments
Near-sightedness and presbyopia. Short-sighted people usually start wearing distance glasses after the age of 40 and take them off when looking at near objects and bringing letters closer to the eyes. This can lead to latent squinting and discomfort. The only way to stop this is to use reading glasses.
Farsightedness and presbyopia. To improve vision, people wear glasses where the initial visual acuity increases by at least 0.5 dioptres as the person ages. Therefore, additional correction is needed for intermediate distances.
Astigmatism and presbyopia. Depending on initial conditions and age, corrections of the astigmatic component are necessary for clear vision at different distances.