Page 185 - Modern Optical Engineering The Design of Optical Systems

168 Chapter Eight

8.4 Defects of the Eye
Nearsightedness (myopia) is a defect of focus resulting from too much
power in the lens and cornea and/or too long an eyeball. The result is
that the image of a distant object falls ahead of the retina and cannot
be focused sharply. Since myopia results from an excessive amount of
positive power, it can be corrected by placing a negative lens before the
eye. The power of the negative lens is chosen so that its image is
formed at the most distant point on which the myopic eye can focus.
For example, a person with 2 diopters of myopia cannot see clearly
1
1
beyond
m (20 in), and a 2 diopter lens (focal length
m or
2 2
20 in) is used to correct for this amount of myopia. The onset of myopia
frequently coincides with adolescence, when growth is most rapid.
Instrument myopia occurs when an observer (especially an
untrained observer) focuses an optical instrument such as a micro-
scope or telescope. There is a tendency to focus the instrument so that
the image appears to be about 20 in (2 diopters) away. This may be due
to the observer’s perception that the image is inside the instrument
and therefore should be nearby. Most experienced observers will focus
an instrument much nearer to an infinity setting. They do this by
moving the microscope toward the object to focus, so that the image is
behind the viewer’s eye (and thus well out of focus) until it is in focus.
Instrument myopia may be related to night myopia, where, in the dark
and with no stimulus, the eye apparently also focuses at a near dis-
tance (60 to 80 in).
Farsightedness (hyperopia) is the reverse of myopia and results from
too short an eye and/or too little power in the refracting elements of the
eye. The image of a distant object is formed (when the eye is relaxed)
behind the retina. Hyperopia can be corrected by the use of a positive
spectacle lens. Obviously farsighted individuals can, to the extent that
their power of accommodation will allow, refocus their eyes to bring the
image onto the retina. If prolonged, this may cause headaches.
Astigmatism is a difference in the power of the eye from meridian to
meridian and usually results from an imperfectly formed cornea, which
has a stronger radius in one direction than in the other. Astigmatism of
the eye is corrected by the use of toroidal surfaces on the spectacle lenses.
Astigmatism “with the rule” has a stronger cornea radius in the ver-
tical meridian than in the horizental meridian.
A contact lens, placed in contact with the surface of the cornea, effec-
tively changes the curvature of the outer surface of the eye (where
most of the visual refractive power occurs). A rigid contact lens can
easily correct astigmatism by replacing the toroidal surface of the
cornea with its own spherical surface. Obviously, a soft (flexible) contact
lens requires an orientation mechanism to align its toroidal power
with that of the eye. Myopia and hyperopia can be corrected with contact