I have lots of cheap eye glasses which it would be good to sort out and
need a more certain way to test the dioptre, than trying them on.

I wear various dioptres for different purposes, reading, TV, driving
etc., and I am rather rough with them, so I have quite a collection.

I'm thinking along the lines of a lamp, then focusing the lamp onto a
surface then measuring the respective distances, but then how to work
out the dioptre?

In article ,
Harry Bloomfield wrote:
I have lots of cheap eye glasses which it would be good to sort out and
need a more certain way to test the dioptre, than trying them on.

I wear various dioptres for different purposes, reading, TV, driving
etc., and I am rather rough with them, so I have quite a collection.

I'm thinking along the lines of a lamp, then focusing the lamp onto a
surface then measuring the respective distances, but then how to work
out the dioptre?

Depending on the quality, you'll often find the strength printed on one of
the legs. But, of course, you'll need another pair to read it...

--
*If PROGRESS is for advancement, what does that make CONGRESS mean?

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

Harry Bloomfield wrote:

how to work out the dioptre?

1/focal length, in metres.

On Thu, 19 Jan 2017 16:39:28 GMT, Harry Bloomfield
wrote:

I have lots of cheap eye glasses which it would be good to sort out
and
need a more certain way to test the dioptre, than trying them on.

I wear various dioptres for different purposes, reading, TV,
driving
etc., and I am rather rough with them, so I have quite a collection.

I'm thinking along the lines of a lamp, then focusing the lamp onto
a
surface then measuring the respective distances, but then how to
work
out the dioptre?

Convex or concave? Could get complicated if there are varifocals
and/or ones that correct for astigmatism too. Opticians do it by
holding the glasses against ones that eliminate the refraction; I
think they call it "neutralising'.

Oh, and a dioptre is the reciprocal of the focal length in metres,
negative for concave.

--
Max Demian

Dave Plowman wrote:

you'll often find the strength printed on one of
the legs. But, of course, you'll need another pair to read it...

Depends ... I take mine off to read little details like that.

It happens that Chris Hogg formulated :
On Thu, 19 Jan 2017 16:39:28 GMT, Harry Bloomfield
wrote:

I have lots of cheap eye glasses which it would be good to sort out and
need a more certain way to test the dioptre, than trying them on.

I wear various dioptres for different purposes, reading, TV, driving
etc., and I am rather rough with them, so I have quite a collection.

I'm thinking along the lines of a lamp, then focusing the lamp onto a
surface then measuring the respective distances, but then how to work
out the dioptre?

You'd be better using sunshine. That's a parallel beam, and measure
the distance from the lens to the point of focus, in metres. Take the
reciprocal and you have the dioptre.

Thanks - not workable at the moment, as there is not much sun about.

On Thursday, 19 January 2017 19:39:32 UTC, Harry Bloomfield wrote:
It happens that Chris Hogg formulated :
On Thu, 19 Jan 2017 16:39:28 GMT, Harry Bloomfield
wrote:

I have lots of cheap eye glasses which it would be good to sort out and
need a more certain way to test the dioptre, than trying them on.

I wear various dioptres for different purposes, reading, TV, driving
etc., and I am rather rough with them, so I have quite a collection.

I'm thinking along the lines of a lamp, then focusing the lamp onto a
surface then measuring the respective distances, but then how to work
out the dioptre?

You'd be better using sunshine. That's a parallel beam, and measure
the distance from the lens to the point of focus, in metres. Take the
reciprocal and you have the dioptre.

Thanks - not workable at the moment, as there is not much sun about.

Is it parallel enough for that to work?


NT

On Thu, 19 Jan 2017 12:01:00 -0800 (PST), wrote:

You'd be better using sunshine. That's a parallel beam, and
measure
the distance from the lens to the point of focus, in metres. Take
the
reciprocal and you have the dioptre.

Thanks - not workable at the moment, as there is not much sun
about.

Is it parallel enough for that to work?

The sun is parallel for most practictical purposes in fact I reckon a
lamp the other side of a room will be good enough. Assuming convex
lenes just focus an image of the lamp and measure the distance lens
to screen. ok for 2 dipotres you might not get a distance of exactly
0.5 m but as lenes are in 0.25 dipoter steps and 1.75 is 0.57m and
2.25 0.44 m it ought to be obvious what the lens is. I'd try it but
we are myopic...

--
Cheers
Dave.

Myself I'd use a local standard. IE why do you need such a technical
measurement? As long as you can grade them as to what you can see with them
does it matter to anyone else. The spec you retaking about assumes a light
source which is a point, like an infinite distance away. I being old
fashioned have no idea how this measurement is actually done, but as I say,
you don't need that, you need a number scheme that you can relate to
yourself when you use them.
Brian

--
----- -
This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please!
"Harry Bloomfield" wrote in message
news
I have lots of cheap eye glasses which it would be good to sort out and
need a more certain way to test the dioptre, than trying them on.

I wear various dioptres for different purposes, reading, TV, driving etc.,
and I am rather rough with them, so I have quite a collection.

I'm thinking along the lines of a lamp, then focusing the lamp onto a
surface then measuring the respective distances, but then how to work out
the dioptre?

On 19/01/2017 21:00, Dave Liquorice wrote:

The sun is parallel for most practictical purposes

That bit of understatement was a very welcome exception.



--
Robin
reply-to address is (intended to be) valid

On Fri, 20 Jan 2017 09:29:14 +0000, Robin wrote:

The sun is parallel for most practictical purposes

That bit of understatement was a very welcome exception.

If sunlight was all truely paralell shadows would have sharp edges,
they don't. This is beacuse the sun is a disc in the sky and you get
rays from opposite sides of that disc.

Now if you took the light of a single star you'd be getting closer.

--
Cheers
Dave.

Brian Gaff laid this down on his screen :
Myself I'd use a local standard. IE why do you need such a technical
measurement? As long as you can grade them as to what you can see with them
does it matter to anyone else. The spec you retaking about assumes a light
source which is a point, like an infinite distance away. I being old
fashioned have no idea how this measurement is actually done, but as I say,
you don't need that, you need a number scheme that you can relate to yourself
when you use them.
Brian

My eyes are just not that sensitive to wearing the wrong dioptre for
the wrong purpose. It takes me a while to realise I am wearing the
wrong ones and I notice the eye strain.

A few years ago, I devised a scheme of colour marking them. Remember
the coloured, numbered as used on cables in electrical control panels?
Those slip neatly onto the arms of the glasses.

The pair I'm wearing now, serve me for most purposes. At the moment I
am (obviously) typing on my laptop and watching TV at the far side of
the room, but they are useless for close up. They sort of work for
longer distances, but I wear a different dioptre for driving.

I try to keep a full range of dioptres in the car, in the caravan and
in the house - hence my confusion.

On 20/01/17 10:05, Dave Liquorice wrote:
On Fri, 20 Jan 2017 09:29:14 +0000, Robin wrote:

The sun is parallel for most practictical purposes

That bit of understatement was a very welcome exception.

If sunlight was all truely paralell shadows would have sharp edges,
they don't. This is beacuse the sun is a disc in the sky and you get
rays from opposite sides of that disc.

Oh dear oh dear..

Now if you took the light of a single star you'd be getting closer.


No comprehension of parallel from rays a non point source is there?

--
Karl Marx said religion is the opium of the people.
But Marxism is the crack cocaine.

On 20/01/2017 10:05, Dave Liquorice wrote:
On Fri, 20 Jan 2017 09:29:14 +0000, Robin wrote:

The sun is parallel for most practictical purposes

That bit of understatement was a very welcome exception.

If sunlight was all truely paralell shadows would have sharp edges,
they don't. This is beacuse the sun is a disc in the sky and you get
rays from opposite sides of that disc.

Now if you took the light of a single star you'd be getting closer.


The fact that the sun is not at infinity makes very little difference to
where a lens produces an (in-focus) image of the sun. Plug the numbers
into the thin lens equation and for typical convex specs the difference
is less than a nanometre. Even a source 5 metres away only shifts the
focus by a few mm from that so as you said a lamp on the other side of a
room is good enough.

--
Robin
reply-to address is (intended to be) valid

On Thursday, January 19, 2017 at 4:39:30 PM UTC, Harry Bloomfield wrote:
I have lots of cheap eye glasses which it would be good to sort out and
need a more certain way to test the dioptre, than trying them on.

I wear various dioptres for different purposes, reading, TV, driving
etc., and I am rather rough with them, so I have quite a collection.

I'm thinking along the lines of a lamp, then focusing the lamp onto a
surface then measuring the respective distances, but then how to work
out the dioptre?

Are they converging lenses (+ dioptres). If so:

1) focus an image of the sun onto a card.
2) measure distance from lens to card in metres.
3) take reciprocal (doptre = 1/ (focal length in metres).

IME you cannot rely on the writing on the frame and in any case it wears off.

Robert

In article ,
Harry Bloomfield wrote:
The pair I'm wearing now, serve me for most purposes. At the moment I
am (obviously) typing on my laptop and watching TV at the far side of
the room, but they are useless for close up. They sort of work for
longer distances, but I wear a different dioptre for driving.

I try to keep a full range of dioptres in the car, in the caravan and
in the house - hence my confusion.

If you need distance correction (for driving etc), that should really be
done properly by an optician. Not like it's that expensive. They will also
be fine for TV - unless you sit on top of the set.

I have contact lenses that correct for distance. But do use ready made for
reading etc. Two strengths are fine for most things. One for this computer
screen and slightly stronger for reading.

And I'm of an age where my accommodation is near zero.

If you are noticing any eye strain, it means something is not right.

--
*I never drink anything stronger than gin before breakfast *

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

"Dave Plowman (News)" wrote in message
...
In article ,
Harry Bloomfield wrote:
The pair I'm wearing now, serve me for most purposes. At the moment I
am (obviously) typing on my laptop and watching TV at the far side of
the room, but they are useless for close up. They sort of work for
longer distances, but I wear a different dioptre for driving.

I try to keep a full range of dioptres in the car, in the caravan and
in the house - hence my confusion.

If you need distance correction (for driving etc), that should really be
done properly by an optician. Not like it's that expensive. They will also
be fine for TV - unless you sit on top of the set.

I have contact lenses that correct for distance. But do use ready made for
reading etc. Two strengths are fine for most things. One for this computer
screen and slightly stronger for reading.

And I'm of an age where my accommodation is near zero.

My accommodation is not as good as it was. I'm very slightly short-sighted:
although I can see fine in the distance and to read road-signs when driving,
my distance glasses just sharpen things up a bit. Driving without my glasses
is not a problem, if I forget them.

My close eyesight, for reading, has got dramatically worse over the past
couple of years (I'm now in my early 50s), and this seems to have started
after I had a heart attack and cardiac arrest: maybe one of the only after
effects of being technically without a pulse for an hour or so (other than
my wife and the ambulance crew giving me CPR) is that my lens muscles have
got weaker. I definitely need reading glasses for anything smaller than
about 12 point print. The computer screen - at a distance of about a metre,
is in between. Without glasses, it's definitely a bit blurred in one eye
(but fine in the other - so I think my brain probably ignores my left eye
for it) whereas my eyes feel as if they are straining if I wear my reading
glasses for the computer. Maybe I'll have to bite the bullet and get a third
pair of in-between glasses for the computer.

When I first found that I needed reading glasses as well as the distance
glasses I already had, I was offered varifocals but I didn't get on at all
well with them. Although I persisted with them for a couple of weeks, I
found that I got weird parallelogram distortion: when I moved my head from
side to side, vertical lines tilted one way or the other, depending on
whether I was panning from right to left or left to right. The optician had
never heard of this side-effect and re-tested me to make sure that she
really had measured the strengths and the exact position of the centre of my
eyes correctly. She then offered me two pairs of single-vision glasses
(reading and distance) at no extra cost, which is a standard arrangement for
those people who can't adjust to varifocals.

On Friday, 20 January 2017 10:05:28 UTC, Dave Liquorice wrote:
On Fri, 20 Jan 2017 09:29:14 +0000, Robin wrote:

The sun is parallel for most practictical purposes

That bit of understatement was a very welcome exception.

If sunlight was all truely paralell shadows would have sharp edges,
they don't. This is beacuse the sun is a disc in the sky and you get
rays from opposite sides of that disc.

Now if you took the light of a single star you'd be getting closer.

Ideally you chose a distance bright gallaxy as they are likely to be further away than a bright star is.

The differnce in accuracy could be huge about the diameter of a hair on a gnats ********, I'd estimate ;-)

In article ,
NY wrote:
My accommodation is not as good as it was. I'm very slightly
short-sighted: although I can see fine in the distance and to read
road-signs when driving, my distance glasses just sharpen things up a
bit. Driving without my glasses is not a problem, if I forget them.

If specs improve your distance vision when driving, you'd be wise to use
them at all times. Even more so in poor visibility when the depth of field
decreases.

--
*Change is inevitable ... except from vending machines *

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

In article ,
NY wrote:
My close eyesight, for reading, has got dramatically worse over the past
couple of years (I'm now in my early 50s), and this seems to have
started after I had a heart attack and cardiac arrest: maybe one of the
only after effects of being technically without a pulse for an hour or
so (other than my wife and the ambulance crew giving me CPR) is that my
lens muscles have got weaker.

It's simply a fact of life. Everyone's accommodation gets worse with age,
although not necessarily at the same speed.

An optician pal said man's natural lifespan is about 35. So you could
still see to make arrows in poor light. ;-)

--
*Why do overlook and oversee mean opposite things?

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

On 20/01/2017 10:09, The Natural Philosopher wrote:
On 20/01/17 10:05, Dave Liquorice wrote:
On Fri, 20 Jan 2017 09:29:14 +0000, Robin wrote:

The sun is parallel for most practictical purposes

That bit of understatement was a very welcome exception.

If sunlight was all truely paralell shadows would have sharp edges,
they don't. This is beacuse the sun is a disc in the sky and you get
rays from opposite sides of that disc.

Oh dear oh dear..

Now if you took the light of a single star you'd be getting closer.


No comprehension of parallel from rays a non point source is there?


Can you try that in English?

On 20/01/2017 11:58, whisky-dave wrote:
On Friday, 20 January 2017 10:05:28 UTC, Dave Liquorice wrote:
On Fri, 20 Jan 2017 09:29:14 +0000, Robin wrote:

The sun is parallel for most practictical purposes

That bit of understatement was a very welcome exception.

If sunlight was all truely paralell shadows would have sharp edges,
they don't. This is beacuse the sun is a disc in the sky and you get
rays from opposite sides of that disc.

Now if you took the light of a single star you'd be getting closer.

Ideally you chose a distance bright gallaxy as they are likely to be further away than a bright star is.

The differnce in accuracy could be huge about the diameter of a hair on a gnats ********, I'd estimate ;-)


Nowhere near that big.

Dave Plowman (News) explained on 20/01/2017 :
If you need distance correction (for driving etc), that should really be
done properly by an optician. Not like it's that expensive. They will also
be fine for TV - unless you sit on top of the set.

I do have them checked, but the correction needed for driving is so
marginal, that the wearing of lenses itself offers only a marginal
advantage versus naked eyeball.


I have contact lenses that correct for distance. But do use ready made for
reading etc. Two strengths are fine for most things. One for this computer
screen and slightly stronger for reading.

I don't like the idea of wearing contacts.


And I'm of an age where my accommodation is near zero.

My accommodation is still pretty good most of the time, I can just
about read without glasses, with a bit of effort.


If you are noticing any eye strain, it means something is not right.

No actual strain, just much easier wearing the correct glasses. I'm
lucky, both of my eyes are identical.

After serious thinking NY wrote :
When I first found that I needed reading glasses as well as the distance
glasses I already had, I was offered varifocals but I didn't get on at all
well with them. Although I persisted with them for a couple of weeks, I found
that I got weird parallelogram distortion: when I moved my head from side to
side, vertical lines tilted one way or the other, depending on whether I was
panning from right to left or left to right. The optician had never heard of
this side-effect and re-tested me to make sure that she really had measured
the strengths and the exact position of the centre of my eyes correctly. She
then offered me two pairs of single-vision glasses (reading and distance) at
no extra cost, which is a standard arrangement for those people who can't
adjust to varifocals.

The first time I began to need multiple pairs, I ordered varifocals. I
just couldn't get on with them at all. I tended to walk about with my
reading glasses on and still do. I found that the varifocals so
distorted the position of things whilst walking I was constantly
tripping up.

In article ,
Harry Bloomfield wrote:
Dave Plowman (News) explained on 20/01/2017 :
If you need distance correction (for driving etc), that should really be
done properly by an optician. Not like it's that expensive. They will also
be fine for TV - unless you sit on top of the set.

I do have them checked, but the correction needed for driving is so
marginal, that the wearing of lenses itself offers only a marginal
advantage versus naked eyeball.

Generally, if a decent optician says you need correction for distance, you
should use that all the time when driving.


I have contact lenses that correct for distance. But do use ready made
for reading etc. Two strengths are fine for most things. One for this
computer screen and slightly stronger for reading.

I don't like the idea of wearing contacts.

I only mentioned that because it means my eyes are corrected for distance.
So allows me to use non prescription reading specs. It's not so convenient
to wear one pair of specs on top of another. ;-)


And I'm of an age where my accommodation is near zero.

My accommodation is still pretty good most of the time, I can just
about read without glasses, with a bit of effort.

If you are slightly short sighted, that makes sense.


If you are noticing any eye strain, it means something is not right.

No actual strain, just much easier wearing the correct glasses. I'm
lucky, both of my eyes are identical.

--
*Be nice to your kids. They'll choose your nursing home.

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

On Fri, 20 Jan 2017 14:12:54 +0000, Dave Plowman (News) wrote:

In article ,
NY wrote:
My accommodation is not as good as it was. I'm very slightly
short-sighted: although I can see fine in the distance and to read
road-signs when driving, my distance glasses just sharpen things up a
bit. Driving without my glasses is not a problem, if I forget them.

If specs improve your distance vision when driving, you'd be wise to use
them at all times. Even more so in poor visibility when the depth of
field decreases.

I had some 'driving specs' made. Just right for driving, but bifocal so I
can see the instruments properly without having to squint and take my
eyes off the road for more than a glance.

The bofical 'line' coincides with the top of the dashboard.
--
My posts are my copyright and if @diy_forums or Home Owners' Hub
wish to copy them they can pay me £1 a message.
Use the BIG mirror service in the UK: http://www.mirrorservice.org
*lightning surge protection* - a w_tom conductor

In article ,
Bob Eager wrote:
If specs improve your distance vision when driving, you'd be wise to
use them at all times. Even more so in poor visibility when the depth
of field decreases.

I had some 'driving specs' made. Just right for driving, but bifocal so
I can see the instruments properly without having to squint and take my
eyes off the road for more than a glance.

That's interesting. I've got no problems reading the instruments on either
of my cars. Perhaps they are further away from me than on some?

Or rather, the only thing I can't read easily is the old rotary odometer
on the Rover. And no chance at night. But all the important ones are OK.

--
*I almost had a psychic girlfriend but she left me before we met *

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

Bob Eager brought next idea :
I had some 'driving specs' made. Just right for driving, but bifocal so I
can see the instruments properly without having to squint and take my
eyes off the road for more than a glance.

The bofical 'line' coincides with the top of the dashboard.

I still have the bifocals, the only bifocals I ever ordered, around
somewhere. I keep intending to see how they might perform for driving.
I don't have a problem reading the dash and the satnav wearing my usual
driving glasses, but I suspect it might be even clearer with bifocals.

On 20/01/17 10:05, Dave Liquorice wrote:
On Fri, 20 Jan 2017 09:29:14 +0000, Robin wrote:

The sun is parallel for most practictical purposes

That bit of understatement was a very welcome exception.

If sunlight was all truely paralell shadows would have sharp edges,
they don't. This is beacuse the sun is a disc in the sky and you get
rays from opposite sides of that disc.

More relevant is the diffusion of light as it passes through the
atmosphere.





Now if you took the light of a single star you'd be getting closer.



--
djc

(–€Ì¿Ä¹Ì¯–€Ì¿ Ì¿)
No low-hanging fruit, just a lot of small berries up a tall tree.

On 20/01/2017 15:46, Harry Bloomfield wrote:
After serious thinking NY wrote :
When I first found that I needed reading glasses as well as the
distance glasses I already had, I was offered varifocals but I didn't
get on at all well with them. Although I persisted with them for a
couple of weeks, I found that I got weird parallelogram distortion:
when I moved my head from side to side, vertical lines tilted one way
or the other, depending on whether I was panning from right to left or
left to right. The optician had never heard of this side-effect and
re-tested me to make sure that she really had measured the strengths
and the exact position of the centre of my eyes correctly. She then
offered me two pairs of single-vision glasses (reading and distance)
at no extra cost, which is a standard arrangement for those people who
can't adjust to varifocals.

The first time I began to need multiple pairs, I ordered varifocals. I
just couldn't get on with them at all. I tended to walk about with my
reading glasses on and still do. I found that the varifocals so
distorted the position of things whilst walking I was constantly
tripping up.

me too

I found they were great straight ahead. I could look at a computer
screen at arms length, and a book under my nose, and have perfect focus
on both.

Trouble is I have two screens at work, and to look at the other one I
had to turn my head - horrible distortion if I tried to use the side of
the glasses.

I really ought to get some bifocals, but poundshop readers work for me.
My distance vision is still OK (albeit not perfect) and quite good
enough to drive with.

Andy

Harry Bloomfield expressed precisely :
I have lots of cheap eye glasses which it would be good to sort out and need
a more certain way to test the dioptre, than trying them on.

I wear various dioptres for different purposes, reading, TV, driving etc.,
and I am rather rough with them, so I have quite a collection.

I'm thinking along the lines of a lamp, then focusing the lamp onto a surface
then measuring the respective distances, but then how to work out the
dioptre?

Now done, but just as a follow up....

Using a light source a worked, but not by trying to measure and use the
reciprocal with a light source a room distance away. Measuring produced
too great an error to be useful.

I ended up using those glasses (spectacles) which were marked with
their dioptre, along with a light source as far away as I could get it,
to make dioptre marks along some masking tape along a bench.

Some of the glasses (not many) didn't focus on the marks, but rather
they focused in between calibration marks - so glasses bought as 2.25
might well be 2.35. The process enabled me to put them all into marked
containers, so I can readily pick out what I need for any purpose.