Refractive States of the Eyes
In considering the refractive state of the eye, we can use either of the following approaches:
The focal point concept: The location of the image formed by an object at optical infinity through a nonaccommodating eye determines the eye’s refractive state. Objects focusing at points anterior or posterior to the retina form a blurred image on the retina, whereas objects that focus on the retina form a sharp image.
The far point concept: The far point is the point in space that is conjugate to the fovea of the nonaccommodating eye; that is, the far point is where the fovea would be imaged if the light rays were reversed and the fovea became the object.
Emmetropia is the refractive state in which parallel rays of light from a distant object are brought to focus on the retina in the nonaccommodating eye (Fig 3-9A). The far point of the emmetropic eye is at infinity, and infinity is conjugate with the retina (Fig 3-9B). Ametropia refers to the absence of emmetropia and can be classified by presumptive etiology as axial or refractive. In axial ametropia, the eyeball is either unusually long (myopia) or short (hyperopia). In refractive ametropia, the length of the eye is statistically normal, but the refractive power of the eye (cornea and/or lens) is abnormal, being either excessive (myopia) or deficient (hyperopia). Aphakia is an example of extreme refractive hyperopia unless the eye was highly myopic (>20.00 D) before lens removal. An ametropic eye requires either a diverging or a converging lens to image a distant object on the retina.
Figure 3-9 Emmetropia with accommodation relaxed. A, Parallel light rays from infinity focus to a point on the retina. B, Similarly, light rays emanating from a point on the retina focus at the far point of the eye at optical infinity.
(Illustration by C. H. Wooley.)
Ametropias may also be classified by the nature of the mismatch between the optical power and length of the eye. In myopia, the eye possesses too much optical power for its axial length, and (with accommodation relaxed) light rays from an object at infinity converge too soon and thus focus in front of the retina (Fig 3-10A). This results in a defocused image on the retina; the far point of the eye is located in front of the eye, between the cornea and optical infinity (Fig 3-10B). In hyperopia, the eye does not possess enough optical power for its axial length, and (with accommodation relaxed) an object at infinity comes to a focus behind the retina, again producing a defocused image on the retina (Fig 3-11A); the far point of the eye (actually a virtual point rather than a real point in space) is located behind the retina (Fig 3-11B).
Astigmatism (a = without, stigmos = point) is an optical condition of the eye in which light rays from a point source on the eye’s visual axis do not focus to a single point. Typically, light rays from a single object point are refracted to form 2 focal lines, perpendicular to each other. Each astigmatic eye can be classified by the orientations and relative positions of these focal lines (Fig 3-12). If 1 focal line lies in front of the retina and the other is on the retina, the condition is classified as simple myopic astigmatism. If both focal lines lie in front of the retina, the condition is classified as compound myopic astigmatism. If, in an unaccommodated state, 1 focal line lies behind the retina and the other is on the retina, the astigmatism is classified as simple hyperopic astigmatism. If both focal lines lie behind the retina, the astigmatism is classified as compound hyperopic astigmatism. If, in an unaccommodated state, one focal line lies in front of the retina and the other behind it, the condition is classified as mixed astigmatism. The orientations of the focal lines reflect, in turn, the strongest and weakest meridians of the net refracting power of the anterior segment refracting surfaces (the cornea and lens). These are referred to as the principal axes.
Figure 3-10 Myopia with accommodation relaxed. A, Parallel light rays from infinity focus to a point anterior to the retina, forming a blurred image on the retina. B, Light rays emanating from a point on the retina focus to a far point in front of the eye, between optical infinity and the cornea.
(Illustration by C. H. Wooley.)
Figure 3-11 Hyperopia with accommodation relaxed. A, Parallel light rays from infinity focus to a point posterior to the retina, forming a blurred image on the retina. B, Light rays emanating from a point on the retina are divergent as they exit the eye, appearing to have come from a virtual far point behind the eye.
(Illustration by C. H. Wooley.)
Figure 3-12 Types of astigmatism. The locations of the focal lines with respect to the retina define the type of astigmatism. The main difference between the types of astigmatism depicted in the illustration is the spherical equivalent refractive error. All of the astigmatisms depicted are with-the-rule astigmatisms—that is, they are corrected by using a plus cylinder with a vertical axis. If they were against-the-rule astigmatisms, the positions of the vertical and horizontal focal lines would be reversed.
If the principal axes of astigmatism have constant orientation at every point across the pupil, and if the amount of astigmatism is the same at every point, the refractive condition is known as regular astigmatism. Regular astigmatism may be classified as with-the-rule or against-the-rule astigmatism. In with-the-rule astigmatism (the more common type in children), the vertical corneal meridian is steepest (resembling an American football or a rugby ball lying on its side), and a correcting plus cylinder is placed with the cylinder axis near 90°. In against-the-rule astigmatism (the more common type in older adults), the horizontal meridian is steepest (resembling a football standing on its end), and a correcting plus cylinder should be placed with the axis near 180°. The term oblique astigmatism is used to describe regular astigmatism in which the principal meridians do not lie at, or close to, 90° or 180°, but instead lie nearer 45° or 135°.
In irregular astigmatism, the orientation of the principal meridians or the amount of astigmatism changes from point to point across the pupil. Although the principal meridians are 90° apart at every point, it may sometimes appear by retinoscopy or keratometry that the principal meridians of the cornea, as a whole, are not perpendicular to one another. All eyes have at least a small amount of irregular astigmatism, and instruments such as corneal topographers and wavefront aberrometers can be used to detect this condition clinically. These higher-order aberrations in the refractive properties of the cornea and lens have been characterized by Zernike polynomials, which are mathematical shapes that approximate various types of irregular astigmatism more closely than the simple “football” model. These aberrations include such shapes as spherical aberration, coma, and trefoil. See Chapters 1 and 7 of this book and BCSC Section 13, Refractive Surgery, for further discussion.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.