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Correcting lenses
11.4.2 In many persons there is a reluctance to wear spectacles because doing so suggests that there is
“something wrong with the eyes” or that “one is getting old”. This natural bias against the use of spectacles
occurs in flight crew, particularly regarding the use of a distance correction. The ever increasing use of
spectacles together with improvements in design and manufacture of spectacle frames and lenses and the
advertising skills of those who make and sell them have made spectacles much more acceptable than was the
case some years ago. Persons mature enough to hold a position of responsibility for control of an aircraft are
usually mature enough to understand that good vision at both distance and near is essential for flight safety.
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-25
Prescription spectacles
11.4.3 With normal uncorrected visual acuity and a good range of accommodation no visual aids are
needed to carry out visual flight deck tasks. However, many flight crew, air traffic controllers and applicants for
these positions do not meet the visual requirements without spectacles or contact lenses, so some knowledge
of these optical devices is useful for the medical examiner. Modern spectacle lenses in the lower powers can
provide excellent, distortion-free correction of the common refractive errors. Unfortunately, as the lens power is
increased the optical aberrations found in all optical systems become significant. These aberrations include
spherical aberration, chromatic aberration, coma3, astigmatism of oblique incidence, field curvature and
distortion. The details of these aberrations are not important but one should know that the degradation of the
imagery can become significant with lens powers greater than 5 dioptres and highly significant with lens powers
greater than 10 dioptres. Apart from these aberrations there are problems which can arise from improper fitting
of spectacles. These include:
a) induced prism effects from tilting of the spectacles or decentration of the lenses so that the
wearer is not looking through the optical centres of each lens
b) incorrect placement of the reading segments in multifocal lenses
c) incorrect distance of the lenses from the wearer’s eyes. The effective power of a lens depends
on its distance from the eye.
11.4.4 Not all refractive errors require correction. A young hyperope with ample accommodation may
have excellent vision at distance and near and will need no correction. Small amounts of astigmatism may not
need correction. Myopia of more than minimal degree will reduce visual acuity at far and require a distance
correction. The decision to prescribe spectacles or contact lenses for an aviator should be made by a vision
care specialist who is familiar with the visual requirements for aviation duties.
11.4.5 A young person requiring distance spectacles will have no problem reading with these spectacles
but when significant presbyopia develops, a different prescription will be necessary for near work.
Management of presbyopia
11.4.6 When the emmetropic subject develops presbyopia, reading spectacles are required. For flight
crew ordinary full-sized lenses are not acceptable because they blur distance vision. The so-called
half-spectacles or “look-over” spectacles are required. In many instances the reading spectacles will not need
to be worn all the time but will be required for looking at charts and maps and during take-off and landing,
especially at night. Such spectacles must be available for immediate use, as required in Annex 1.
11.4.7 A myopic person will develop presbyopia as anyone else, but can usually cope quite well by taking
his (distant vision) spectacles off when he needs to read. As this is not acceptable for flight crew when flying,
some sort of multifocal correction is required.
11.4.8 The hyperope will develop symptoms of presbyopia earlier than persons with other types of
refractive error because some of the accommodative reserve is used to compensate for the hyperopia. This
situation will require a multifocal correction.
3 Coma: an aberration of the image formed of a point-source off the principal axis of an optical system. It
consists of a spreading out of the image in a plane roughly at right angles to the optic axis producing a
comet-like tail.
III-11-26 Manual of Civil Aviation Medicine
11.4.9 It should be noted that, in accordance with Annex 1, a pilot shall demonstrate that one pair of
spectacles is sufficient to meet the visual requirements. The use of separate distance and reading spectacles
is not acceptable because of possible problems when having to change from one set to another during a critical
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Manual of Civil Aviation Medicine 2(46)
