Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

INTRODUCTION
Myopia is a very common condition in humans, typically associated to age,
genetic factors, and visual habits. The prevalence rate in adults is more than 70% in
some Asian populations, and around 25% in western countries. Emetropia (absence of
refractive errors) demands a fine tuning between the power of all the ocular
components of the eye and the axial length (distance between the cornea and the
retina). The growing eye has an active control mechanism to tune the eye elongation to
the optical power of the optical elements of the eye. This process is called
emetropization, and when it is disrupted, leads to myopia in most cases.
1.3.2. Presbyopia
As age increases, the lens movement is reduced, and accommodation is gradually
diminished. The first symptoms are usually noticed at around the age of 40, when
visual tasks at near distances become more difficult. The ability to accommodate is
completely lost at 55 (Weeber and van der Heijde, 2007). This condition is called
presbyopia (Fig. 1.3).
Although several potential causes for presbyopia have been hypothesized, the best
supported is the progressive loss of elasticity of the lens produced by the continuous
growing of the nucleus (inner part of the lens), blocking the capacity of changing its
shape and its refractive index distribution (Glasser and Campbell, 1998, Charman,
2008).
a)
Relaxed
Far Point
Accomodation
Range
Near Point
Accomodated
Both retinal
images in focus
b)
Presbyopic
eye
Defocused
retinal image
Fig. 1.3. Accomodation and presbyopia on a myopic eye (for an emmetropic eye the
far point would be at infinity). a) In the non presbyopic eye the cristalline lens
changes its power to keep objects at different distances in focus at the retina, a
process called accommodation. b) In the presbyopic eye the ability to accommodate is
partially or totally lost, the cristalline lens does not change its power and therefore
different object distances correspond to different defocus states at the retina.
Even when the ability to accommodate is completely lost, the depth of focus of
the eye provides certain capacity to observe objects at varying distances (i.e. there is a
range for which the eye cannot appreciate the blur introduced by defocus). This depth
of focus is highly dependent on the pupil size, on the amount of aberrations and on the
criterion used to measure or estimate it, and is around 0.5 D for the human eye
(Tucker and Charman, 1975, Atchison et al., 1997).
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