"NY" wrote in message
...
"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.

I didn’t wear them when reading books etc and I am short sighted.

They *may* also be able to read without those glasses because their lens
can focus at that distance without correction.

I certainly could and still can, tho now the book is quite close to my nose.

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).

Those I know who have had cataract surgery do use glasses for reading.

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.