The Eyeballs

Optical image formation is by a pair of very simple optical units (the eyeballs) onto detector matrices at the back of the eyeballs (the retinas). The optical units are very simple lenses, with adjustable focal length (by muscular compression) to provide adjustment of focusing distance (accommodation) but without any attempt to provide corrections for such things as the common lens aberrations. In particular there is no correction for chromatic aberration. This means that, apart from anything else, the longitudinal image position varies widely across the visible spectrum for a given accommodation setting (a differential focus change of around two dioptres from red to blue), this feature being particularly noticeable for the blue end of the spectrum.

In addition, because of the simple nature of the lenses, the image formed on the retina is surprisingly blurred, even for best focus, as compared with the images formed by conventional, fully-corrected man-made optical systems.

The amount of light entering the eyeball is controlled by the iris, which acts like an automatic aperture control on cameras to provide a rapid adjustment of light reaching the back of the eyeball in response to external light levels (see Chapter 2.2 of 'Vision and Acquisition' for more information).