It is clear, moreover, that one can define
'normal vision' in functional, rather than
statistical, terms, and that it is qualitatively
different from all forms of faulty vision. The
'standard' of 20/20 is often held to be
arbitrary, and even meaningless, it being clear
that many people can get along perfectly well
with less while many others can achieve far
higher levels of acuity. While this is true as
far as it goes, the fact is that there is a
definite correspondence between the Snellen 20/20
standard and the optical resolution limit of
individual cone cells, which indicates that
Snellen's empirical calculation comes remarkably
close to the most modern findings as to the
acuity level to be expected of healthy eyes
functioning normally in reasonable conditions.
Higher acuities would normally require some form
of 'software enhancement' in the brain, and lower
levels would point to an abnormality in the eyes
themselves, or in their use.
So, our idea of the perfection that we may
wish to achieve needs to take all this on board.
Normal vision is inherently stable and self -
regulating (homeostatic), whereas faulty vision
is always prone to deteriorate. This means that,
for any patient who has had normal vision and
lost it, there has been a definite critical point
at which there has been a change from normal to
abnormal function, well before a loss of acuity
has been apparent. This needs to be recognised by
optical practitioners, who at present seem to
think it sufficient to correct the acuity,
without concerning themselves with function.
For a vision teacher, however, the question is,
are we always to attempt to achieve normal
eyesight (as we have now defined it)? Is it
"OK" to have deficient eyesight if it
is appropriate to your personality/situation or
is feeling comfortable with 'deviant' eyesight,
in itself pathological? Are we to aim for
perfection, or to be content with encouraging
improvement, and how far are the two
possibilities compatible?
At first sight, there is no contradiction. After
all, if the use and function are improved, the
acuity will generally follow, to a greater or
lesser extent. But we need to recognise that,
however much improvement is gained, unless the
vision is actually normal, we can assume that
functional problems remain leading to a
consequent tendency to deteriorate.
