Accommodation of the eye is the ability of the human eye to see sharply all objects which are located at various distances in front of the eye depending on the changes of massiveness of the optical system of the eye. The mechanism of eye accommodation is not the same for all animals. For example fish accommodates through the change of position of the lens, some types of birds accommodate through the increase of curvature of the cornea and protraction of the human eye. As far as humans are concerned accommodation is caused by the increased curvature of anterior area of the eye lens while at the same time its thickness also changes. Accommodation is usually the same on both eyes.
For the vision to be sharp it is necessary that the image is located on the retina. The eye is able to see sharply objects at various distances. As it was mentioned before adaptation of the human eye to various distances is called accommodation. The change of shape and thereby also the parameters of the eye lens are caused by ciliary muscles. When observing objects located close to us the radius of curvature of anterior area of the eye lens decreases and at the same time its thickness increases which results in reduction of the focal distance of the optical system of the human eye. (When observing distant objects the ciliary muscles are strained less whereas when observing objects located close to us they are tense and the eye gets tired)
The mechanism of eye accommodation
To put it simply we can say that eye accommodation leads to contraction of fibers of the ciliary muscle which leads to relaxation of the lens and thereby the change of the radius of curvature. However, whole process is substantially more complicated. The basic biomechanical and anatomical changes during accommodation include: contraction of the ciliary muscle, the ciliary muscle with the choroid and posterior part of the ciliary suspension move forward by 0.5 mm, anterior part of the ciliary zonule relaxes.
Elastic eye lens and its casing become round and the refrangibility increases because the equatorial diameter of the lens decreases by 0.4 mm, anterior pole of the lens moves forward by 0.3 mm and changes the radius of curvature from 11 to 5.5 mm, posterior pole moves forward by 0.15 mm and changes the radius of curvature from 5.18 to 5.05 mm. The central thickness of the eye lens increases by 0.36 to 0.58 mm and as a consequence of gravity the lens falls by 0.3 mm.
The process of accommodation of the human eye is therefore influenced by two factors: the ability of the lens to change its shape and the strength of the ciliary muscle. The ability of the lens to change its shape (physical deformation of the lens – compressibility and elasticity) is called physical accommodation. Contraction strength of the ciliary muscle is then called physiological accommodation.
Accommodation of the eye is influenced by many other factors including optical (spherical and chromatic aberration, astigmatism) and non-optical ones (the size, the distance). Even mood and will but also illumination and contrast have some influence.