The eyes, even to the color of the iris, are fundamentally the same in all people, and of practically the same size, but their apparent size is considerably altered by the opening of the lids, which in turn is influenced by the formation of the upper lid and the cushion of fat surrounding the eyes at the back.
Fritz Lange, professor of orthopedics ("Die Sprache des menschlichen Antlitzes"), has distinguished six basic types of lid formation, a formulation which is of great use in portrait painting. The general tone of a person is often indicated by the eyelids. The eyes can be made large at will, but if the lids are usually rather dropped it is a sign of weak tone.
The upper lid may, of course, relax momentarily with anyone - for instance, in moments of strong and passive sensual excitement, listening to music, or breathing in intense perfumes. Wide eyes have always been associated with a deep spiritual life, and there is a constant tendency to portray eyes larger than normal in relation to the rest of the face and body.
The width of the space between the lids does not differ much between individuals, but there is a much larger difference in the space between the two inner corners of the eyes. Here again the constitution is as much responsible as the position of the eye sockets in the skull. Deep-set eyes generally lie close together. The normal distance between the two inner corners of the eyes corresponds to the breadth of the visible eyeball.
The most important thing in drawing an eye is to establish the position of the eyeball from the placing of the iris and its pupil. The iris is immovable behind the cornea; the eyeball is moved by six muscles. The circular iris has a concentric circular opening in the middle, the pupil, which can be enlarged and reduced by muscular action, to regulate the amount of light penetrating the eye. Emotion as well as light affects the size of the pupil. Depression, anger, and sorrow make it larger so that objective vision is disturbed by excessive penetration of light. "To see things in a false light" is an apt figure of speech. Animation reduces the size of the pupil.
Focusing is operated by a lens lying behind the iris and moved by the ciliar muscle. The changes in it cannot be seen directly; yet it is always possible to know if someone is looking at an object or through it, or seeing in a "visionary" manner. There are no rules to help in drawing this; close observation is the only way. Shading and high lights, the path of the look of each eye, and their combined direction, which is parallel only when looking into the far distance, are the things to notice.
The much admired portait which seems to look at the beholder from whichever direction it is viewed is achieved by the trick of making both eyes look straight out of the picture perpendicular to the picture plane. The color of the iris has no connection with character. The permanent expression of the eyes is related to the position of the iris in the eyeball and the position and shape of the lids. The "expression" of the eyes is, in fact, the general facial expression. Tears, or an increase of tear fluid, make the eyes shine more.