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c) Mixed astigmatism
e) Compound hyperopic astigmatism
a) Compound myopic astigmatism
d) Simple hyperopic astigmatism
b) Simple myopic astigmatism
III-11-16 Manual of Civil Aviation Medicine
Irregular astigmatism
11.3.47 When the principal meridians are not at right angles, the astigmatism is called irregular. Irregular
astigmatism occurs when there has been corneal scarring from any cause and in the developmental
abnormality keratoconus. It is not possible to correct irregular astigmatism fully using spectacle cylinders.
Contact lenses provide the best chance of optimum correction because the inner surface of the contact lens
replaces the irregular surface of the eye as one of the refracting surfaces in the optical system.
11.3.48 All optical systems, including the eye, have aberrations. These include:
a) Chromatic aberration due to the different amount of refraction of the different wavelength
components of white light. Long wavelengths are refracted less than short wavelengths.
b) A variety of optical aberrations including spherical aberration, coma (the unsharp halo which
can result from objects being off-centre), astigmatism of oblique incidence, field curvature and
distortion.
11.3.49 In low power lenses these aberrations are minimal but in the higher power lenses, say above plus
or minus 5 dioptres, they become increasingly important so that distortion and alteration of visual field are of
concern in the aviation environment. Improvements in lens design and manufacture such as high index, thin
lenses have reduced the distortion in the higher power lenses but contact lenses provide better visual fields and
less distortion than strong spectacle lenses and should be considered in applicants with large refractive errors.
Anisometropia and aniseikonia
11.3.50 Difference in refractive error between the two eyes is anisometropia. Correction of anisometropia
produces a difference in retinal image size in the two eyes. When this difference in size is perceived by the
person, it is called aniseikonia (from Gr. eikon = image, likeness, picture).
11.3.51 Large amounts of anisometropia can be fully corrected with spectacles in children, but in adults
correction of more than 3 dioptres of anisometropia may be problematic. Tolerance of an anisometropic
spectacle correction and the induced aniseikonia varies greatly between individuals. Applicants with significant
amounts of anisometropia should be evaluated by a vision care specialist.
Substandard vision in one eye
11.3.52 It is common to see applicants in whom one eye meets the required standards of Annex 1 but
whose other eye cannot be corrected to the required standards because of amblyopia or other eye disease.
Such applicants require evaluation by a vision care specialist to determine the cause of the vision loss. They
may be assessed as fit under the provisions of Annex 1, 1.2.4.8. In doubtful cases a medical flight test to
evaluate visual performance during flight might be appropriate.
Near visual acuity and accommodation
11.3.53 In most modern aircraft a major part of the flying time is spent evaluating information displayed
within the cockpit. Cockpit information systems become ever more complex and the need to see clearly at
various distances inside the cockpit is just as important as the need for good distance acuity. Aeronautical
charts, head-up displays, colour-coded warning lights, radio dials, topographical mapping and weather radar
displays are some of the things which the aviator must see clearly and which require good visual acuity at close
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-17
and intermediate ranges.
11.3.54 In the young eye the lens is pliable and through the action of the ciliary muscle can easily increase
its curvature so as to provide the necessary increase in power to focus on close objects. This ability to
accommodate diminishes with age as the lens becomes increasingly rigid — a condition called presbyopia. The
power of accommodation is measured while the applicant wears distance correction if prescribed. Small print
which can just be read at arm’s length is used and the applicant reads the print while the chart is moved
towards the eyes until a point is reached when the print starts to become blurred. The applicant is encouraged
to put maximum effort into the test. The distance from the eyes at which the print first becomes blurred is the
near-point of accommodation. The reciprocal of this distance in metres is the accommodative amplitude in
dioptres. Instead of using the ordinary near vision test card, a near-point rule can be used and has the
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Manual of Civil Aviation Medicine 2(42)
