I am short sighted so anything at arms length or beyond is *completely* out of focus.
Just noticed though that if I hold a pair of reading glasses ( not mine as I don’t need them for close up vision) at arms length the frame and everything around is out of focus as usual but the image coming through the lens of the background scene is
in complete 20:20 sharp focus .Albeit upside down.
Cant quite figure out how this works. Is it something to do with the focal length in my eye between the lens and retina?

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

Lou <noelturntive@live.co.uk> wrote:

I am short sighted so anything at arms length or beyond is *completely* out of focus.
Just noticed though that if I hold a pair of reading glasses ( not mine
as I don’t need them for close up vision) at arms length the frame and everything around is out of focus as usual but the image coming through
the lens of the background scene is in complete 20:20 sharp focus .Albeit upside down.
Cant quite figure out how this works. Is it something to do with the
focal length in my eye between the lens and retina?

You‘ve invented the telescope. ;)

The reading glasses are forming a real image (aka an aerial image) between
them and your eyes. Because the glasses’ focal length is short, the image
is far enough away that you can focus on it.

If you draw a ray diagram, you will see why the image is rotated 180 degrees—inverting in both x and y directions is equivalent to a half turn.

Cheers

Phil Hobbs

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

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Thursday, 2 November 2023 at 16:33:24 UTC, Phil Hobbs wrote:

Lou <noeltu...@live.co.uk> wrote:

I am short sighted so anything at arms length or beyond is *completely* out of focus.
Just noticed though that if I hold a pair of reading glasses ( not mine
as I don’t need them for close up vision) at arms length the frame and everything around is out of focus as usual but the image coming through the lens of the background scene is in complete 20:20 sharp focus .Albeit upside down.
Cant quite figure out how this works. Is it something to do with the
focal length in my eye between the lens and retina?

You‘ve invented the telescope. ;)

The reading glasses are forming a real image (aka an aerial image) between them and your eyes. Because the glasses’ focal length is short, the image is far enough away that you can focus on it.

If you draw a ray diagram, you will see why the image is rotated 180 degrees—inverting in both x and y directions is equivalent to a half turn.

Thanks for that. It made me think about the possibility of if I had a bigger lens
I could watch my tv (upside down and reversed) lying down on the sofa
without my glasses.

Cheers

Phil Hobbs

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

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Thursday, 2 November 2023 at 16:33:24 UTC, Phil Hobbs wrote:

Lou <noeltu...@live.co.uk> wrote:

I am short sighted so anything at arms length or beyond is *completely* out of focus.
Just noticed though that if I hold a pair of reading glasses ( not mine
as I don’t need them for close up vision) at arms length the frame and everything around is out of focus as usual but the image coming through the lens of the background scene is in complete 20:20 sharp focus .Albeit upside down.
Cant quite figure out how this works. Is it something to do with the
focal length in my eye between the lens and retina?

You‘ve invented the telescope. ;)

The reading glasses are forming a real image (aka an aerial image) between them and your eyes. Because the glasses’ focal length is short, the image is far enough away that you can focus on it.

If you draw a ray diagram, you will see why the image is rotated 180 degrees—inverting in both x and y directions is equivalent to a half turn.

Cheers

Phil Hobbs

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

Here’s another odd effect that maybe you can shine a light on.
I don’t know if you’ve noticed this effect, it’s probably not that good for my eyes.
If I look a the reflection of the sun off of a kitchen ornament which is essentially
a circular strip of reflective metal I see a hilite of the reflected sun. Around it I see an aura that looks a lot like those images of magnetic fields on iron filings.
https://en.wikipedia.org/wiki/Magnetic_field#/media/File:Magnet0873.png

I presume the ”aura” is something to do with the light being
dispersed by the material of the retina or maybe the fluid in the optical cavity.

Anyways to cut a long story short if I rotate my head the magnetic field
image in the aura doesn’t rotate. It’s always at the same angle relative to the
reflection. And thus relative to the sun.
Am I seeing the angle of polarisation of the suns reflected light after its been
Plane polarised by the reflection? Kind of like an optical version of
a calcite crystal used by the vikings.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)