"The Natural Philosopher" wrote in message
...
Weasel. That was not the question that was asked

"Do they set both eyes to a fixed infinity?"

The answer is of course no.

Not necessarily "of course". I would expect the lens to be set to a distance
that gives good vision at some useful distance without needing to wear
glasses. And the two choices are probably infinity so you only need to wear
glasses for reading and other close work (maybe needing a selection of
different glasses for different distances) or else reading, so you only need
to wear glasses for longer distance such as driving and walking around
outside.

No one is stupid enoght to believe the question was 'do they set the eyes
the same?' Of course they do. Why would they not?

Because if they set one eye to infinity and the other to reading distance
then *if the brain can cope with it* you have good vision at both ends of
the distance range without needing glasses for either. My question was "is
that feasible or can the brain not cope with ignoring whichever image is out
of focus?". I'm guessing that it isn't, otherwise people who haven't had a
cataract replaced would wear glasses that had lenses of different strength
so a single pair of glasses, worn all the time, will cater for all
situations without the need for multiple pairs, bi/tri-focals or
vari-focals. (*)


What is the normal distance that cataract lenses are set to - ie the one
that most people choose if they have a choice? Infinity?

I'm surprised at the range of distances that my mum's cataract lenses can
cope with: she only needs glasses for reading, and can see acceptably well
from about 10 feet to infinity. Are modern cataract lenses capable of
adjusting slightly using the same muscles that previously adjusted the eye's
own lens, or are they still a fixed focal length?


(*) As an aside, why do some people cope better with vari-focals than
others? When I first needed reading glasses, having previously only needed
very weak distance glasses, I was offered vari-focals. The optician offered
me a deal which gave the same price for either one pair of vari-focals or
two separate single-vision glasses. After the sight test and the
measurements of the exact locations of my pupils (because vari-focals depend
critically on looking through the exact centre of the lens) I was given the
vari-focals. And the effect was *horrendous*, even after wearing them for
several days to give my eyes/brain chance to get used to them. There was an
objectionable amount of parallelogram distortion: if two vertical lines
moved across my field of view, they went from \\ to || to //. Likewise, if I
moved my head slightly while looking at the same object, which requires the
eyes to swivel to keep the object fixed, I got the parallelogram distortion.
The optician did another sight test and re-checked the eye separation
measurements, and all was in agreement with the previous tests. So they
offered me the fall-back solution of separate distance and reading glasses
at no extra cost.