"Dave Plowman (News)" wrote in message
...
I'm sure many can read
adequately without reading specs at all stages of life.
'
Only if they start out with at least one eye 'short sighted'.

I presume short-sighted younger people (who still have working "focussing
muscles" and lenses which obey those muscles) will have glasses which they
wear all the time to correct a *systematic* error - eyeball too large (or is
it too small) for the lens to focus at infinity. They *may* also be able to
read without those glasses because their lens can focus at that distance
without correction.

As people get older, their focussing muscles become weaker and/or their lens
becomes stiffer and less able to change from its relaxed (infinity) setting
to focus at a closer distance which requires the lens to be compressed
diammetrically so it becomes thicker from front to back and so has a shorter
focal length.

As you say, if one lens is short-sighted from an early age, it will be able
to provide one-eyed close vision even when the other non-short-sighted eye
can no longer change focus to see close up.

I've always wondered what eye surgeons do when they replace cataracts in
both lenses? Do they set both eyes to a fixed infinity (and require the
person to wear glasses to read) or do they set one to infinity and the other
to much closer (so as to cover both close and distance in different eyes).
Does the brain get used to discarding whichever eye's image is blurred and
only use whichever eye is providing an in-focus image?

I don't know because my eyes have always had very similar focal lengths -
for many years I was very slightly short-sighted and got a very small
benefit from wearing weak distance glasses for driving. Now I'm in my
fifties both eyes are losing their ability to focus to close distances so I
need reading glasses; interestingly my distance glasses now actually make
even distance less sharp than with my unaided eyes, and they definitely make
my closer vision (eg of the dashboard) worse - so I've stopped wearing the
distance glasses for driving. It's scary how my reading glasses used to be
needed only when reading and made the computer screen more blurred than
unaided, whereas now my eyes have changed further and my reading glasses are
needed even for computer screen - and indeed with my reading glasses I can't
focus as close as I could, so I probably need a new stronger prescription.