Abstract
A number of axes are recognized in the eye, such as optic axis, visual axis, pupillary axis, fixation axis, and line of sight [1,2], as shown in Fig. 1. It is well known that the visual axis is the most important for ophthalmic prescription, measurement of refraction and corneal configuration and so on [3] (Fig. 2). The visual axis is defined as the two lines, one from the fixation point to the first nodal point in object space, and other from the fovea to the second nodal point in image space [1]. However, objective measurement of the visual axis is extremely difficult because it is imaginary. In clinical viewpoint, the pupillary axis and line of sight are well in use because of the simplicity of measurement. Strictly speaking, these axes differ fairly from the visual axis, and do not pass through the fovea.
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