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3.3.5 "Emmetropization" towards Myopia
The title of this section sounds very controversial, as emmetropia is in contrast to myopia (see
section 1.3.1). This issue is, however, more complex (see Figure 8 below):
Figure 8 The shift of the accommodation effort caused by myopia
• Literally the word emmetropization means that the eye is going to the state of emmetropia
(see section 1.3.1), i.e. to a state where proper focusing of distant objects can be
achieved with a relaxed accommodation.
• Further the word emmetropization
is used to
describe the ability of the
eye to adjust the eyegrowth
in length during
development for optimal
optical imaging. Wildsoet
et al. stated255: "...when
optical vergence information
is restricted to
one plane, this plane becomes
the end-point of
emmetropization." and
"That eyes may emmetropize
to distances
other than real infinity is
not a new observation ...
laboratory-raised animals
tend to be more myopic
... when their environment
is purposefully restricted".
In other words,
emmetropization is used
as well as the name of a
mechanism, which adjusts
the eye length according
to the preferred
viewing distance.
In the history of human development both definitions matched, because mostly the dominating
viewing distance was the far distance.
Today, with often excessive near work, both definitions of emmetropia are controversial:
If the eye adjusts to the dominating viewing distance it will often become not emmetropic,
but myopic! Therefore, "emmetropization" in this case does not lead to emmetropia, but to
myopia!
Emmetropization in the sense of an adjustment of the length of the eye to achieve proper vision
for "the individual, mostly used viewing distance" is necessary when considering the individually
different optical parameters of the eye before emmetropization takes place. Emmetropization
has to be seen as a very smart mechanism to balance for inborn optical differences
and deficiencies. |
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