ot optics question for Phil

[John Larkin]

What's a good way to measure the negative diopter of my glasses, given
minimal tools up the the cabin?

I can let the sun shine through a lens and see an enlarged circle on
the ground a few feet away. What's the math on that?



--

John Larkin Highland Technology, Inc trk

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

[a a]

Optical Diopter Strength Explained

https://focusers.com/pages/explanation-of-diopter-strength

[John Larkin]

On Thu, 11 Aug 2022 17:32:40 -0700 (PDT), a a <mant...@gmail.com>
wrote:

Thanks, but that doesn't answer my question.

[a a]

https://wiki.endmyopia.org/wiki/Diopters

[John Larkin]

On Thu, 11 Aug 2022 16:38:38 -0700, John Larkin
<jjla...@highlandtechnology.com> wrote:

>
>What's a good way to measure the negative diopter of my glasses, given
>minimal tools up the the cabin?
>
>I can let the sun shine through a lens and see an enlarged circle on
>the ground a few feet away. What's the math on that?

I found it online.

https://physics.stackexchange.com/questions/566207/how-to-measure-the-strength-of-a-prescription-eyeglass-lens

My lens is 2" wide and sunlight projects a 4" wide image at 11" from a
screen. So the fl of the lens is -11".

That's bad, since that eye focuses at about 17". These glasses are
over-correcting.

[Cydrome Leader]

John Larkin <jjla...@highlandtechnology.com> wrote:
> On Thu, 11 Aug 2022 16:38:38 -0700, John Larkin
> <jjla...@highlandtechnology.com> wrote:
>
>>
>>What's a good way to measure the negative diopter of my glasses, given
>>minimal tools up the the cabin?
>>
>>I can let the sun shine through a lens and see an enlarged circle on
>>the ground a few feet away. What's the math on that?
>
> I found it online.
>
> https://physics.stackexchange.com/questions/566207/how-to-measure-the-strength-of-a-prescription-eyeglass-lens
>
> My lens is 2" wide and sunlight projects a 4" wide image at 11" from a
> screen. So the fl of the lens is -11".
>
> That's bad, since that eye focuses at about 17". These glasses are
> over-correcting.

You're pulling some weird NASA moves by asking "what's the diopter of a
lense" and then concluding a focal length of -11 inches.

Next question is where the hell did you get those glasses in the first
place, if you have no idea of what their prescription even is?

[John Larkin]

On Fri, 12 Aug 2022 04:40:54 -0000 (UTC), Cydrome Leader
<pres...@MUNGEpanix.com> wrote:

>John Larkin <jjla...@highlandtechnology.com> wrote:
>> On Thu, 11 Aug 2022 16:38:38 -0700, John Larkin
>> <jjla...@highlandtechnology.com> wrote:
>>
>>>
>>>What's a good way to measure the negative diopter of my glasses, given
>>>minimal tools up the the cabin?
>>>
>>>I can let the sun shine through a lens and see an enlarged circle on
>>>the ground a few feet away. What's the math on that?
>>
>> I found it online.
>>
>> https://physics.stackexchange.com/questions/566207/how-to-measure-the-strength-of-a-prescription-eyeglass-lens
>>
>> My lens is 2" wide and sunlight projects a 4" wide image at 11" from a
>> screen. So the fl of the lens is -11".
>>
>> That's bad, since that eye focuses at about 17". These glasses are
>> over-correcting.
>
>You're pulling some weird NASA moves by asking "what's the diopter of a
>lense" and then concluding a focal length of -11 inches.

That conversion is not difficult.

>
>Next question is where the hell did you get those glasses in the first
>place, if you have no idea of what their prescription even is?

I don't have the prescription handy. And I like to measure things.

I think my eye has changed since I got the glasses.

Are you always so rude?

[Joe Gwinn]

On Fri, 12 Aug 2022 04:40:54 -0000 (UTC), Cydrome Leader
<pres...@MUNGEpanix.com> wrote:

>John Larkin <jjla...@highlandtechnology.com> wrote:
>> On Thu, 11 Aug 2022 16:38:38 -0700, John Larkin
>> <jjla...@highlandtechnology.com> wrote:
>>
>>>
>>>What's a good way to measure the negative diopter of my glasses, given
>>>minimal tools up the the cabin?
>>>
>>>I can let the sun shine through a lens and see an enlarged circle on
>>>the ground a few feet away. What's the math on that?
>>
>> I found it online.
>>
>> https://physics.stackexchange.com/questions/566207/how-to-measure-the-strength-of-a-prescription-eyeglass-lens
>>
>> My lens is 2" wide and sunlight projects a 4" wide image at 11" from a
>> screen. So the fl of the lens is -11".
>>
>> That's bad, since that eye focuses at about 17". These glasses are
>> over-correcting.
>
>You're pulling some weird NASA moves by asking "what's the diopter of a
>lense" and then concluding a focal length of -11 inches.

Nothing weird here. Just some simple optics.

This well predates NASA, having been developed by Isaac Newton in the
16th century:

.<https://en.wikipedia.org/wiki/Opticks>


Diopters are the inverse of the focal length in meters. Yes, a lens
can have negative power expressed in diopters:

.<https://en.wikipedia.org/wiki/Thin_lens>


Joe Gwinn

[Phil Hobbs]

John Larkin wrote:
> On Thu, 11 Aug 2022 16:38:38 -0700, John Larkin
> <jjla...@highlandtechnology.com> wrote:
>
>>
>> What's a good way to measure the negative diopter of my glasses, given
>> minimal tools up the the cabin?
>>
>> I can let the sun shine through a lens and see an enlarged circle on
>> the ground a few feet away. What's the math on that?
>
> I found it online.
>
> https://physics.stackexchange.com/questions/566207/how-to-measure-the-strength-of-a-prescription-eyeglass-lens
>
> My lens is 2" wide and sunlight projects a 4" wide image at 11" from a
> screen. So the fl of the lens is -11".
>
> That's bad, since that eye focuses at about 17". These glasses are
> over-correcting.
>

You'll get a better measurement by making the aperture much smaller than
the focal length. (The magnitude of the FL, that is.)

An object at a distance d_o from the center of a thin lens of focal
length f produces an image at a distance d_i, where

1/d_o + 1/d_i = 1/f.

The sun is very distant, so we can ignore 1/d_0, so that the image of
the sun is at f, which for a concave lens is on the side towards the sun.

You figure the FL using similar triangles as usual, except
backwards--given two projections, find the focal point. That takes one
more line of algebra.

The sun 's angular subtense is about 0.5 degrees, so you have to take
that contribution out somehow--if you measure the disk diameter out to
where the intensity falls to nearly zero, your measured angle will be
0.5 degrees too large, and your measured FL correspondingly too short.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

[Cydrome Leader]

Joe Gwinn <joeg...@comcast.net> wrote:
> On Fri, 12 Aug 2022 04:40:54 -0000 (UTC), Cydrome Leader
> <pres...@MUNGEpanix.com> wrote:
>
>>John Larkin <jjla...@highlandtechnology.com> wrote:
>>> On Thu, 11 Aug 2022 16:38:38 -0700, John Larkin
>>> <jjla...@highlandtechnology.com> wrote:
>>>
>>>>
>>>>What's a good way to measure the negative diopter of my glasses, given
>>>>minimal tools up the the cabin?
>>>>
>>>>I can let the sun shine through a lens and see an enlarged circle on
>>>>the ground a few feet away. What's the math on that?
>>>
>>> I found it online.
>>>
>>> https://physics.stackexchange.com/questions/566207/how-to-measure-the-strength-of-a-prescription-eyeglass-lens
>>>
>>> My lens is 2" wide and sunlight projects a 4" wide image at 11" from a
>>> screen. So the fl of the lens is -11".
>>>
>>> That's bad, since that eye focuses at about 17". These glasses are
>>> over-correcting.
>>
>>You're pulling some weird NASA moves by asking "what's the diopter of a
>>lense" and then concluding a focal length of -11 inches.
>
> Nothing weird here. Just some simple optics.

It is weird. He's going on about diopters, but they're not a part of
either known or unknowns in what he's trying to calculate or confirm.

I have no issue with inch focal lengths by themselves. Tons of "standard"
sizes in mm for objective lenses are based off the original ones in
inches.

[Phil Hobbs]

Eyeglasses prescriptions are in dioptres.

Don't worry, CD, JL is very unlikely to crash into Mars this week. ;)

[John Larkin]

On Fri, 12 Aug 2022 18:24:25 -0000 (UTC), Cydrome Leader

Glasses are prescribed in diopters. I measured the focal length of
that lens as -11 inches. That's about -3.5 diopters.

Is that weird?

[Cydrome Leader]

He doesn't know his rx, or that of the glasses. It's all pointless.

> Don't worry, CD, JL is very unlikely to crash into Mars this week. ;)

Ha.

[Phil Hobbs]

If the resting focus of that eye is +2.3 dioptre (17 inch FL), then a
-3.6 dioptre corrective is indeed too strong. (This is after cataract
surgery, so there's no accommodation to take into account.)

[John Walliker]

On Friday, 12 August 2022 at 19:24:32 UTC+1, Cydrome Leader wrote:
> Joe Gwinn <joeg...@comcast.net> wrote:
> > On Fri, 12 Aug 2022 04:40:54 -0000 (UTC), Cydrome Leader
> > <pres...@MUNGEpanix.com> wrote:
> >
> >>John Larkin <jjla...@highlandtechnology.com> wrote:
> >>> On Thu, 11 Aug 2022 16:38:38 -0700, John Larkin
> >>> <jjla...@highlandtechnology.com> wrote:
> >>>
> >>>>
> >>>>What's a good way to measure the negative diopter of my glasses, given
> >>>>minimal tools up the the cabin?
> >>>>
> >>>>I can let the sun shine through a lens and see an enlarged circle on
> >>>>the ground a few feet away. What's the math on that?
> >>>
> >>> I found it online.
> >>>
> >>> https://physics.stackexchange.com/questions/566207/how-to-measure-the-strength-of-a-prescription-eyeglass-lens
> >>>
> >>> My lens is 2" wide and sunlight projects a 4" wide image at 11" from a
> >>> screen. So the fl of the lens is -11".
> >>>
> >>> That's bad, since that eye focuses at about 17". These glasses are
> >>> over-correcting.
> >>
> >>You're pulling some weird NASA moves by asking "what's the diopter of a
> >>lense" and then concluding a focal length of -11 inches.
> >
> > Nothing weird here. Just some simple optics.
> It is weird. He's going on about diopters, but they're not a part of
> either known or unknowns in what he's trying to calculate or confirm.
>

Well, the diopter is just the reciprocal of the focal length in metres, so
it is relevant. Opticians use diopters because lenses can be stacked
in front of each other and the powers in diopters simply add or subtract
as appropriate. This is much easier than working directly with focal lengths.

> I have no issue with inch focal lengths by themselves. Tons of "standard"
> sizes in mm for objective lenses are based off the original ones in
> inches.

John

[jla...@highlandsniptechnology.com]

It's just like resistors in parallel.

[Cydrome Leader]

When is the last time you use mhos in a calcuation?

[jla...@highlandsniptechnology.com]

On Mon, 15 Aug 2022 18:26:44 -0000 (UTC), Cydrome Leader

I type

rval
1/x

(RPN) often. I don't call them mhos; they are actually Siemens.


What resistor do you put across 5 ohms to get 3 ohms?

3
1/x
5
1/x
-
1/x

[Cydrome Leader]

Seems like the answer is never. I see lots of repetetive 1 over
calculations.

There really is no going back once you learn RPN. Even a fancy calculator
where you can see rows of nested braces just doesn't compare.

I always though mhos were just odd. So when did you last use susceptance,
in siemens of course? Maybe it's time to hawk special "ultra high
susceptance" cables to the audiophiles.

[Jeroen Belleman]

Conductance, susceptance, admittance, they have their uses. In the
design of matching networks, filters and pulse forming networks, it's
common to switch back and forth between those and their reciprocals
all the time.

Jeroen Belleman

[John Larkin]

On Tue, 16 Aug 2022 21:06:12 -0000 (UTC), Cydrome Leader

I have an app on my Android phone, Free42, which looks and feels a lot
like a classic HP calculator. Very nice.

Are kids learning RPN these days?

[Cydrome Leader]

Do you have an example? I'm just curious. Say you're making a 2 stage type
E network, where do the weird units come into play?

[Cydrome Leader]

Not directly. They push python as a programming language, and it's real
backwards, but no official RPN classes anywhere. TI still runs the
cartel for school approved graphing calculators.

I'm not even sure what they teach these days that offers any real value
outside of constantly taking tests ina school, and learning how to be meek
and offended by everything.

Was chatting with a machinist who say they regularly have people apply for
assembly jobs at the factory and can't even use a socket wrench. They pass
on those folks. People are so helpless fewer and fewer cars even come with
a spare, as they know the driver won't be able to change it in the first
place. Sad times.

[jla...@highlandsniptechnology.com]

On Wed, 17 Aug 2022 04:12:52 -0000 (UTC), Cydrome Leader

I have a relative who has a PhD in Engineering Mechanics. He is hugely
offended if anyone calls him a mechanical engineer. He's helpless at
fixing things.

[Jeroen Belleman]

The typical case is the development of a complex impedance into a ladder
network. A continuous fraction expansion has alternating impedance and
admittance expressions as you descend into the denominator. Look up
writings by Wilhelm Cauer, Sid Darlington, Ernst Guillemin and Ronald
Foster, among many others.

Jeroen Belleman

[Phil Hobbs]

Right, which is why most Smith chart pads have both impedance and
admittance graticules.

I use admittance all the time in doing mental arithmetic for circuit design.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net

https://hobbs-eo.com

[Phil Hobbs]

Bet he's not as useless as a guy I used to know who has a PhD in
"Industrial Engineering".

What is "industrial engineering", anyway? AFAICT it seems to be a
dumping ground for people who wash out of ME or EE.

[Lasse Langwadt Christensen]

they did the math, a spare is used so rarely that it is a waste of space and
fuel to carry one around

[Cydrome Leader]

Who did the math? Airbgs and seatbelts seem like a waste too by the same
MBA logic.

[Lasse Langwadt Christensen]

you can't be silly enough to think that is the same....

[John Larkin]

On Mon, 3 Oct 2022 20:37:09 -0000 (UTC), Cydrome Leader

If you read old books, up to 1960s maybe, "punctures" were very
common. Nowadays tires rarely go flat, or even lose much air.

[John Walliker]

I have changed wheels a few times over the last decade. The causes of
punctures were horseshoe nails, fencing nails, woodscrews and in one case
a damaged cast iron drain cover.

I do drive a lot on country roads adjacent to stables and horse breeders.

John

[Martin Brown]

Rubber chemistry and tyre manufacture has improved a lot in that time.

The other snag is that modern cars push tyres hard enough that they have
a spin handedness so your choice it limited to carrying a generic thin
space saving spare or runflats. The latter leaves more room in the boot.

Some of us are on run flats which are hellishly expensive but very
impressive if you really do have a blowout at motorway speeds. Driver
action required is pretty much limited to continue to steer, slow down
and pull over to take a look at the damage. You can limp home at 50mph
for up to 50 miles on a flat run flat tyre. Although I was inclined to
give it a break every 15 minutes or so since it runs very hot.

Doing 50mph on the inside lane of a UK motorway is pretty scary with
HGVs on their limiters at 56mph overtaking at the last possible moment.

I have had about half a dozen flat tyres and two of them were motorway
blowouts after hitting coach bolts/sharp metal shards on the road. One
went right through the outside tyre wall the other did invisible damage
to inside tyre wall - only visible once it was up on a ramp. The run
flat experience was much more user friendly than the classic tyre.

--
Regards,
Martin Brown