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Dioptre Setting: Fallacy and Fact (1 Viewer)

tenex

reality-based
Relying on wikipedia.de, I believe that's the original 1855 Helmholtz explanation, which since has been expanded (1980s-ish, it seems) by identifying some muscles that support accommodation towards infinity. I believe that would imply a finite (but long) distance for relaxed focussing, but the article only mentions this aspect in passing.
If you stop to think about it (apologies to Helmholtz who may not yet have known about evolution?), having the relaxed focus at infinity seems an unlikely result when most important events and tasks involve much shorter ranges. Also it would be difficult to make corrections for any reason if there were no further adjustment possible in that direction.

There was some discussion (even measurement!) of this here a couple of years ago. As I recall it varies individually, in a range of around 1-2m. Search this thread for "resting focus": New Horizons II
 
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Tringa45

Well-known member
Europe
There was some discussion (even measurement!) of this here a couple of years ago. As I recall it varies individually, in a range of around 1-2m. Search this thread for "resting focus": New Horizons II
When someone describes their own post as "inspirational" I raise an eyebrow. And I don't think the PhD could have had anything to do with optics ;)!
 

Hauksen

Forum member
Antarctica
Hi Tenex,

If you stop to think about it (apologies to Helmholtz who may not yet have known about evolution?), having the relaxed focus at infinity seems an unlikely result when most important events and tasks involve much shorter ranges. Also it would be difficult to make corrections for any reason if there were no further adjustment possible in that direction.

There was some discussion (even measurement!) of this here a couple of years ago. As I recall it varies individually, in a range of around 1-2m. Search this thread for "resting focus": New Horizons II

Thanks, that looks quite interesting! I actually came across the "no stimulus" focus when reading up for this thread, but I don't think it's necessarily a given that "no stimulus" and "no muscle contraction" coincede. As (asymmetrical) muscle contraction is what Bill wants to minimize, if I understand him correctly, the "no stimulus" focus might not be of interest in this particular context. (However, I haven't read through that multi-page thread, so I might have it all wrong!)

Regards,

Henning
 

WJC

Well-known member
If you stop to think about it (apologies to Helmholtz who may not yet have known about evolution?), having the relaxed focus at infinity seems an unlikely result when most important events and tasks involve much shorter ranges. Also it would be difficult to make corrections for any reason if there were no further adjustment possible in that direction.

There was some discussion (even measurement!) of this here a couple of years ago. As I recall it varies individually, in a range of around 1-2m. Search this thread for "resting focus": New Horizons II
"... having the relaxed focus at infinity seems an unlikely result when most important events and tasks involve much shorter ranges."

If that is a statement attributed to me, whoever said it's so is WRONG. I did not say it. But others keep bringing INFINITY into the conversation. Starting off with a relaxed focus for viewing any distance, even across the street, is the goal. The less you know ... the more it matters!

When I was doing optics for Ft. Lewis, I noticed that some of the night-vision gear (usually the AN-PVS-14) had large numeral painted on the side. Once when a platoon leader came in to pick up his repaired gear, I asked him about it. He said some of his soldiers wanted to be sure they got the same device for every patrol they went on!!!

I had to make the assumption that they weren’t smart enough to turn a large, well-numerated focus knob (see attached). I’ll fear for our nation the next time we get into a real war.
 

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Tringa45

Well-known member
Europe
"... having the relaxed focus at infinity seems an unlikely result when most important events and tasks involve much shorter ranges."

If that is a statement attributed to me, whoever said it's so is WRONG. I did not say it. But others keep bringing INFINITY into the conversation. Starting off with a relaxed focus for viewing any distance, even across the street, is the goal. The less you know ... the more it matters!
Bill,
For the human eye there is no practical difference between "across the street" and infinity. Most eye doctors measure at 20 ft or 6 m and that is a difference to infinity of less than 0,2 dioptres.
Viewed through a telescope or binocular though, you would have to multiply that difference by the square of the magnification.

John
 

WJC

Well-known member
Bill,
For the human eye there is no practical difference between "across the street" and infinity. Most eye doctors measure at 20 ft or 6 m and that is a difference to infinity of less than 0,2 dioptres.
Viewed through a telescope or binocular though, you would have to multiply that difference by the square of the magnification.

John
That's why this incredibly simple subject requires so much verbiage. I didn't mean to imply that one's dioptric strengths changed at all! But that the best, most "fiddle Free" focus for ANY DISTANCE would be attained when the binocular was focused when the eyes were relaxed.
 
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Binastro

Well-known member
Hi John,

I have five glasses.

Normal distance glasses, set at 20ft.
Astro glasses set at 120 metres plus. I can easily see the difference between distance and astro glasses.
T.V. glasses set at about 7ft.
Computer glasses.
Reading glasses.

I also have a sixth spectacle that I use for window shopping that is a fixed plus 2 dioptre.
As both eyes have different prescriptions I get increased depth of focus.
But this gets tiring to use.

In addition, when I need extra magnification I use various glasses held 8 inches in front of my eyes.

With a Nikon multicoated achromat of 700mm focus I get 2.4x magnification held at arms length at 30 metres and 2.8x when viewing at 7 metres.
The image is beautifully clear and sharp.

Regards,
B.
 

WJC

Well-known member
Hauksen wrote:

"In Germany, there's a magazine discussing military matters that goes by the title "Soldiers and Technology". In the Bundeswehr, you can't ever mention the title without everyone nearby responding, in unison, "Two worlds collide!"

Allow me to offer the cover of a US Army training manual.
 

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Maljunulo

Well-known member
There are frequent misunderstandings and misconceptions on dioptre setting and many are of the opinion that the dioptre should be set on an object at or near infinity. Conversely, would they expect the dioptre setting to be wrong for objects at shorter distances?
On another thread I asserted that the distance of a target chosen for dioptre setting was irrelevant, and in expectation of contradiction decided to conduct an experiment.

Firstly though, it is important to establish that a telescope or binocular is an afocal instrument. Focus is achieved by the human eye on the retina, just like that of a camera lens on the film or sensor. The"focusser" of a binocular is there to place the image of the viewed object at an apparent distance that is comfortable to the observer, regardless of the real distance of the object.
For the normally sighted this would be at infinity, where the eyes are most relaxed and the rays from any point on the viewed object would emerge parallel from the eyepiece. For the near-sighted the rays would diverge and for the far-sighted they would converge.
The dioptre adjustment compensates for any differences in the observer's eyes, left and right. Observers wearing glasses would normally set the dioptre to zero.

John

PS:- Sorry for the title, Bill. I just couldn't resist :).
This is probably the best explanation I recall ever reading.

(Or that I recall offhand, but my memory gets a bit worse every year)
 
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tenex

reality-based
If that is a statement attributed to me, whoever said it's so is WRONG. I did not say it.
No Bill, Helmholtz did... and apparently the idea was accepted for a long time.

Exactly where the eye's resting focus is might not seem to matter in the use of afocal instruments, but perhaps the fact that it's so close helps to explain why we have such issues with diopter setting? Anything we're looking at while bringing binoculars to our eyes for adjustment is probably a good deal farther away than that, so the eyes are initially likely not to be relaxed and trying to correct that can help.
 
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WJC

Well-known member
No Bill, Helmholtz did... and apparently the idea was accepted for a long time.

Exactly where the eye's resting focus is might not seem to matter in the use of afocal instruments, but perhaps the fact that it's so close helps to explain why we have such issues with diopter setting? Anything we're looking at while bringing binoculars to our eyes for adjustment is probably a good deal farther away than that, so the eyes are initially likely not to be relaxed and trying to correct that can help.
A week or so ago, I thought of discussing Schmidt-Pechan vs. Abbe-Koenig prisms. However, since this simple subject of at-rest focusing has taken on a weird little life of its own ... I thought better of it!
 

WJC

Well-known member
Moments ago, I learned of losing a friend and mentor. Dr. John Greivenkamp of Center for Optical Sciences at the UA and the most recent past president of SPIE. He was 3 years younger than me.
 

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Trinovid

Well-known member
United States
The diopter adjusts one side of the binocular to match the other, as seen by that particular user. You set it once and forget it...it is not something you fiddle with.
To be honest, I think this is all that really needs to be said..........to someone who has never used binoculars before!
 

Patudo

Well-known member
So as I understand the procedure, it works like this:

  • Set diopter knob to 0
  • Prefocus binoculars on a distant target for left eye (both eyes open)
  • Focus unaided left eye on a distant target (both eyes open)
  • Raise binoculars quickly to both eyes (both eyes open)
  • Quickly use centre focus to bring target into focus for right eye (both eyes open)
  • Read dpt difference between both eyes off center focus wheel.

As my center focus wheels don't have a dpt scale, I suspect that's not quite right - maybe you could clarify?

Many thanks in advance!

Regards,

Henning

Mate... what exactly is this rigmarole supposed to achieve? ...
 

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