Spherical aberration. Projected images are distorted due to the curvature of the retina.

Chromatic aberration. Images are blurred due to the various degrees of refraction of spectral colours.

The process of perception is frequently explained by comparing the eye with a camera. In the case of the camera, an adjustable system of lenses projects the reversed image of an object onto a film. The amount of light is controlled by a diaphragm. After developing the film and reversing the image during the enlarging process, a visible, two-dimensional image of the object becomes apparent. Similarly, in the eye, a reversed image is projected onto the retina of the eye via a deformable lens. The iris takes on the function of the diaphragm, the light-sensitive retina the role of the film. The image is then transported via the optic nerve from the retina to the brain, where it is adapted in the visual cortex and made available to the conscious mind.
In regard to the eye, however, there are considerable differences between what is actually perceived and the image on the retina. The image is spatially distorted through its projection onto the curved surface of the retina. Through chromatic aberration - light of various wavelengths is refracted to varying degrees, which produces coloured rings around the objects viewed. These defects, however, are eliminated when the image is being processed in the brain.