Eye 63 Diopters

16.1.1 Uncorrected and Corrected Visual Acuity

Uncorrected visual acuity: This refers to the resolving power of the eye without corrective lenses.

Corrected visual acuity: This refers to the resolving power of the eye with an optimal correction provided by corrective lenses (determined by visual acuity testing).

Both uncorrected visual acuity and corrected visual acuity provide information on how far apart two objects must be for the eye to perceive them as distinct objects (minimum threshold resolution). For the eye to perceive two objects as distinct, at least one unstimulated cone must lie between two stimulated cones on the retina. The cone density is greatest in the center of the retina and central visual acuity is highest. There the cones are spaced only 2.5 ^m apart. This interval increases toward the periphery of the retina, and both uncorrected visual acuity and corrected visual acuity decrease accordingly. Cone spacing and physical effects such as diffraction and optical aberrations limit the average minimum threshold resolution, the minimum visual angle to one minute of arc (the individual maximum value is approximately 30 seconds of arc). One minute of arc is 1/60 of a degree or approximately 0.004 mm, which is somewhat more than the width of a cone. This corresponds to the maximum resolving power of the retina (Fig. 16.1).

16.1.2 Refraction: Emmetropia and Ametropia

Refraction is defined as the ratio of the refractive power of the lens and cornea (the refractive media) to the axial length of the globe. Emmetropia is distinguished from ametropia.

Emmetropia (normal sight): The ratio of the axial length of the eye to the refractive power of the cornea and lens is balanced. Parallel light rays that enter the eye therefore meet at a focal point on the retina (Figs. 16.2 and 16.6 a) and not anterior or posterior to it, as is the case in ametropia.

— Resolution of the eye (minimum threshold resolution).

— Resolution of the eye (minimum threshold resolution).

Fig. 16.1 Two points (O1 and O2) can only be perceived as distinct if at least one unstimulated cone (z) lies between two stimulated cones (xandy) on the retina. Due to optical aberrations and diffraction, a punctiform object is reproduced as a circle (k). This results in a maximum resolution of the eye of 0.5-1 minutes of arc or 0.5/601/60 of a degree. The drawing is not to scale.

Fig. 16.1 Two points (O1 and O2) can only be perceived as distinct if at least one unstimulated cone (z) lies between two stimulated cones (xandy) on the retina. Due to optical aberrations and diffraction, a punctiform object is reproduced as a circle (k). This results in a maximum resolution of the eye of 0.5-1 minutes of arc or 0.5/601/60 of a degree. The drawing is not to scale.

— Focal point in emmetropia and ametropia.

— Focal point in emmetropia and ametropia.

Fig. 16.2 Parallel rays of light entering the eye from an optically infinite distance meet at a focal point on the retina in emmetropia (black lines). In hyperopia, this focal point (II) lies posterior to the retina (green lines). In myopia (I), it lies anterior to the retina (red lines).

Ametropia (refractive error): There is a mismatch between the axial length of the eye and the refractive power of the lens and cornea. The ametropia is either axial, which is common, or refractive, which is less frequently encountered. The most common disorders are nearsightedness, farsightedness, and astigmatism.

Very few people have refraction of exactly ±0.0 diopters. Approximately 55 % of persons between the ages of 20 and 30 have refraction between +1 and -1 diopters.

Emmetropia is not necessarily identical to good visual acuity. The eye may have other disorders that reduce visual acuity, such as atrophy of the optic nerve or amblyopia.

The refractive power of an optical lens system is specified in diopters, which are the international units of measure. Refractive power is calculated according to the laws of geometric optics. According to Snell's law, the refraction of the incident light ray is determined by the angle of incidence and difference in the refractive indices n of the two media (Table 16.1).

The maximum total refractive power of an emmetropic eye is 63 diopters with an axial length of the globe measuring 23.5 mm. The cornea accounts for 43 diopters and the lens for 10-20 diopters, depending on accommodation. However, the refractive power of the eye is not simply the sum of these two values. The optic media that surround the eye's lens system and the distance between the lens and cornea render the total system more complex.

The refractive power D (specified in diopters) of an optical system is the reciprocal of the focal length of a lens f (specified in meters). This yields the equation: D = 1/f.

Example: Where a lens focuses parallel incident light rays 0.5 m behind the lens, the refractive power is 1/0.5 m = +2 diopters. This is a converging lens. Where the virtual focal point is in front of the lens, the refractive power is 1/-0.5 m = -2 diopters. This is a diverging lens (Fig. 16.3).

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