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c) astigmatism
d) combinations of the above.
Hyperopia
11.3.41 The hyperopic eye is deficient in refractive power so that when it is not accommodating, parallel
light rays from a distant object are not refracted sufficiently to be focussed on the retina. Distant objects will be
blurred unless the person is able to use his accommodation to add the necessary refractive power. In young
eyes there is ample accommodative power to compensate for significant amounts of hyperopia but as
presbyopia develops this accommodative reserve diminishes. Thus a 20-year-old with 5 dioptres of hyperopia
may need no spectacle correction to see well in the distance but at age 60 the same person will require almost
full correction of the refractive error to see distant objects clearly.
11.3.42 The relationship between hyperopia and convergent strabismus will be discussed later.
Figure 11-4 illustrates hyperopia and how it is corrected using a plus power lens.
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-13
Figure 11-4. Correction of hyperopia — plus sphere
Myopia
11.3.43 In myopia the eye has too much refracting power so that parallel light rays are focussed in front of
the retina resulting in blurred distance vision. Light coming from near objects is diverging, and if the distance
between the object and the eye matches the amount of myopia, the near object will be in focus. For example,
an eye with 3 dioptres of myopia will see objects at a distance of 1/3 m clearly without accommodating. Bear in
mind that an eye with uncorrected visual acuity of 6/6 may be slightly myopic and with appropriate correction
may have an acuity of 6/3 or twice as good. Figure 11-5 illustrates myopia and shows how correction is
achieved with minus power lenses.
Astigmatism
11.3.44 Astigmatism is defined as the inability of an optical system to form a point image of a point object.
It results from different curvatures of the refracting surfaces of an optical system, including the eye. In an optical
system with no astigmatism the curvature of each refracting surface is the same in all planes which is to say
that the curvature in the horizontal plane (the 180-degree axis) is the same as the curvature in the vertical plane
(90-degree axis). Such a surface is said to be spherical. If the curvature of the refracting surface is not the same
in all planes the surface is said to be toric (from L. torus = swelling, bulge, knot), and there will be astigmatism.
One way to visualize this is to think of the surface of an orange as spherical while the surface of a lemon would
be toric.
11.3.45 In clinical optics the different planes of the refracting surfaces are called meridians. In a toric
surface there will be one meridian with a maximum curvature and one with a minimum curvature. These are
called the principal meridians. If the principal meridians are at right angles (e.g. at 90 and 180 degrees or at
45 and 135 degrees) the astigmatism is said to be regular. Regular astigmatism in the eye can be corrected
with cylinders. If the principal meridians are not at right angles, the astigmatism is said to be irregular. Irregular
astigmatism cannot be fully corrected with spectacle cylinders but it can often be corrected with contact lenses.
Uncorrected
Corrected
III-11-14 Manual of Civil Aviation Medicine
Figure 11-5. Correction of myopia — minus sphere
11.3.46 Many eyes have some regular astigmatism. The amount and orientation of the astigmatism is
indicated by the cylindrical component of the spectacle correction. Figure 11-6 shows an astigmatic refracting
system and illustrates how the astigmatism results in two focal lines rather than a point focus which would be
the case if there were no astigmatism. In the astigmatic eye, the position of these two focal lines with regard to
the retina is used to classify the astigmatism as follows:
a) If both focal lines are in front of the retina there is compound myopic astigmatism
b) If one focal line is in front of the retina and the other is on the retina there is simple myopic
astigmatism
c) If one focal line is in front of the retina and the other is behind the retina there is mixed
astigmatism
d) If one focal line is on the retina and the other is behind the retina there is simple hyperopic
astigmatism
e) If both focal lines are behind the retina there is compound hyperopic astigmatism.
a) Myopic eye looking at a distant object
b) Myopic eye corrected for distant vision
c) Myopic eye looking at a near object
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-15
Figure 11-6. Types of astigmatism
Vertical
Vertical
Vertical
Vertical
Vertical
Horizontal
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