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Fig. 17.8
An encircling scleral buckle constricts the globe and increases the axial length.
silicone oil compared with the crystalline lens is more strongly converging and therefore produces a myopic shift. The hypermetropia which normally results from aphakia will therefore be of the order of only +4.00 to +6.00 D.
Gas in the posterior segment of a phakic eye greatly increases the refractive power of the posterior surface of the lens and causes a large myopic shift; this may allow indirect ophthalmoscopy without the use of a condensing lens. Gas filling an aphakic eye makes the posterior corneal surface highly diverging and almost neutralises the refractive power of the cornea to allow an unaided view of the fundus.
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Appendix I
Mathematical definitions
1. Sine, Cosine and Tangent.
2. Angles formed by parallel lines
The angles α formed by parallel lines AB and CD intersecting line EF are all equal.
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Appendix II
Specifications of various indirect ophthalmoscopy lenses. |
|
|
|
Lens |
Angular magnification |
Field of view |
Laser spot magnification |
20D |
2.97 |
46° |
0.34 |
28D |
2.16 |
55° |
0.46 |
78D |
0.87 |
73° |
1.15 |
90D |
0.72 |
69° |
1.39 |
Superfield |
0.72 |
120° |
1.39 |
Larger diameter lenses give a larger field of view.
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Index
A
aberrations, 89 chromatic, 89
duochrome test dependent on, 90–92 coma, 97
curvature of field, 97–8
ocular curvature of field, 98 image distortion, 97
barrel, 98 pin-cushion, 98
oblique astigmatism, 95 multifocal lens and, 95
spherical, 92 correction/reduction of, 92–3
aplanatic surfaces, use of 92–3 lens form adjustment, 92 pupil controlling, 94
'stops', use of, 92
ocular spherical aberration, 93 reduction by
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corneal curvature, 93;
lens nuclear refractive index, 94; 'stopping' effect of pupil, 94; Stiles–Crawford effect, 94
absolute refractive index, 34 determination by refractometer, 34 optical density and, 33–4
accommodation, 107–11 amplitude, 109
decline with age, 143 far point, in absence of, 109
near point, with maximum, 109 range of, 109
accommodative convergence/accommodation ratio (AC/A), 110–11 achromatic lens systems, 89–90
Airy disc, 10, 11 amblyopia
anisometropia, significant cause of, 116 ametropia
ametropic error, 120 definition of, 113
far point of eye in, 119–20 optical correction of, 119
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optics of, 113–40
relative spectacle magnification in, 126 spectacle magnification, 126
angle, critical, 38
relation to total internal reflection, 38–9 aniseikonia, 133
correction of, 134 anisometropia, 116
amblyopia, significant result of, 116 bifocals, not tolerated in, 147 definition of, 116
anterior chamber
angle, visualisation of, 39–40 depth, pachometer and, 207
aphakia,
see also hypermetropia refractive correction by
contact lens, 133 intra-ocular lens, 133–4
multifocal, 138 power of, 134–7
spectacle, 124
image distortion, 130
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image magnification, 126 optical problems with, 130
relative spectacle magnification, 128–30 ring scotoma with, 131–2
iseikonic lens and, 134 optical constants in, 103 unilateral correction of, 133
aniseikonia with, 133 apostilb, 23
aplanatic surface, 93 applanation tonometer, 204–206
principles of working, 205 A-scan ultrasonography, 137 aspheric lens, 93–4, 176 astigmatism, 114
axis determination, 73, 232 cross-cylinder, uses in, 71–3 stenopaeic slit in, 117–18
irregular, 115 oblique, 115 regular, 115
surgical correction of, see under refractive surgery, 242
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varieties of, 115–16 automated clinical refraction, 210
basic principles, 210 early optometers, 213
infrared optometers, 214 meridional refractometry, 212 optometer principle, 210 Scheiner principle, 210–11
photoscreening, 214–15
B
back vertex distance, 101, 125 back vertex power, 81, 100–101 bifocal lenses
cemented wafer, 144 fused, 145
solid (executive), 146
split bifocal lens, Franklin design, 144
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