In article ,
NY wrote:
"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.

Quite. Some advantage being born short sighted if you can't have perfect
eyes from birth. As at least you will still be able to read in later life
without correction. I'm so called long sighted, so need correction for
distance at all times - and additional correction for reading.

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 simply dunno. I wear contact lenses and did try one made to my reading
presciption. But didn't like it, being used to having both eyes for
everything before.

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.

Because I have my distance vision corrected by contact lenses I obviously
need reading specs at my advanced age. And less powerful ones for the
computer. And even more powerful ones for soldering etc - or looking for
PCB faults.

--
*Cleaned by Stevie Wonder, checked by David Blunkett*

Dave Plowman London SW
To e-mail, change noise into sound.