Ametropia: Definition

Introduction

An eye is defined as ametropic if its far point does not lie at infinity. An (infinitely) far object point is then no longer imaged as a point on the retina. If the cornea and the crystalline lens have spherical surfaces, identical optical conditions are present in all meridian planes, and the eye is spherically ametropic.

If, however, the refracted rays only converge in two meridian planes (principal meridians) perpendicular to each other, the eye is termed astigmatically ametropic.

Kinds of Ametropia

Myopia: An eye is defined as myopic if its far point is located at a real finite distance in front of it. Far point refraction is negative: K < 0. The myopic eye usually has an overall length which is too long in comparison with the refractive power of the average emmetropic eye (axial myopia). Occasionally, it has a refractive power F_R which is too high in relation to the overall length of the standard eye (refractive myopia). The image-side focal point F′_eyeof the eye...