Cross-sectional view of the human eye
The image formed inside the human eye is inverted. But the information sent to the brain would be corrected. Hence an eye can see things as they really are.
The most remarkable optical instrument known is the eye. Light enters the eye through the transparent cover called the cornea, which does
about 70% of the necessary bending of the light before the light passes through
the pupil (which is an aperture in the iris). The light then passes through the
lens, which is used only to provide the extra bending power needed to focus images
of nearby objects on the layer at the back of the eye, the retina.
The lens is held by ciliary muscles, which stretch and relax to change the focal
length of the lens. The distance of the image that a lens produces on the retina
depends on the object distance and the focal length of the lens. In the case of
the eye lens, however, the image is always produced on the retina no matter where
the object is. In other words, for the eye lens, the image distance is fixed. Therefore
the focal length of the eye lens must change in accordance with the object distance.
This is the function of ciliary muscles.