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....but life is cruel so gradually you will lose accommodation as you eye's lens stiffens and by the time you're 30 or 40 you will definitely want a focusing mechanism on your binoculars.

Perhaps binoculars should come in different models, just like pet food for different aged pets. I think I qualify for the senior citizen model....
 
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The reason DOF appears shallow in binoculars and scopes is because ...

Hi Henry,

Thank you for your explanation. It is an interesting perspective and might be the correct reason why binoculars provide a shallow depth of field. However, we have not fully solved the mystery yet. ;) What I like to understand is that why a healthy human eye can not focus on individual objects located at various distances while looking through binoculars? I am not saying why those objects don't all appear sharp. I am asking why it is not possible to look at individual image points and see them in sharp focus?

To use a camera analogy, the COTAX AX camera with a 50mm f/1.4 lens mentioned above can focus on any point in a scene (say from 1m to infinity). If the aperture is at f/1.4, other points slightly before or after the object on which the camera has focused will appear blurry. But it is possible to focus on any desired distance.

Why can't human eye -when positioned behind an eyepiece- do the same? Why can't we focus on various objects in front of our binoculars and see a shallow-depth-of-field view of only the object we are concentrating on?

-Omid
 
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Old 20x50 Porros can be a problem, but not much for me. I find them very useful as night as much fainter stars are visible than in a 10x50.

B.

Since we are dealing with a "point source", magnification has no effect on the size of the image formed on the retina.

Post #77 enplanes why a faint star could be more visible in a 20X50 binoculars (exit light beam diameter 2.5mm) compared to 10X50 (exit light beam diameter 5mm). Same amount of input light is concentrated in a narrower pencil of light when using 20X50 leading to increased perceived brightness.
 
Hi Henry,

Thank you for your explanation. It is an interesting perspective and might be the correct reason why binoculars provide a shallow depth of field. However, we have not fully solved the mystery yet. ;) What I like to understand is that why a healthy human eye can not focus on individual objects located at various distances while looking through binoculars? I am not saying why those objects don't all appear sharp. I am asking why it is not possible to look at individual image points and see them in sharp focus?

To use a camera analogy, the COTAX AX camera with a 50mm f/1.4 lens mentioned above can focus on any point in a scene (say from 1m to infinity). If the aperture is at f/1.4, other points slightly before or after the object on which the camera has focused will appear blurry. But it is possible to focus on any desired distance.

Why can't human eye -when positioned behind an eyepiece- do the same? Why can't we focus on various objects in front of our binoculars and see a shallow-depth-of-field view of only the object we are concentrating on?

-Omid

Doesn't Henry's description already explain this? My understanding is the binocular presents to the eye a single distance at a time with its associated depth of field and the eye can only accomodate and examine this image. To examine other objects at different distances the binocular has to have its point of focus adjusted so that that eye is presented with another selection or slice of the view which it can then work with.

The example of the Contax with its 50mm lens only works because its magnification isn't high enough to be comparable to 8x or 10x binos.

I am happy to be proved wrong but at the moment I feel that the culprit is the magnification and this is what Henry described.

Lee
 
Yes, Henry did explain this, and very well too. His "sloppy, ball park only" tests are also highly illuminating.

In Omid's example of the Contax camera, focusing the lens does what the human eye pupil will do if you have young eyes and no extra magnification in front of it.
 
I also think Henry got it right. While the source of light from the binocular going to the eye can be Omid's point source, no matter what the point source is, 6x, 8x, 10x, it is still a point source many times more powerful than the 1x of our DNA modelled optical system that is dealing with the magnified point source. As Henry pointed out, we are left with our natural accommodation. When the limits of that accommodation are reached, focus on the binocular seems to become necessary. That seems further obvious that the closer the magnification gets to 1 x, the increasing depth of focus makes focusing less necessary.
 
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Sub millimetre pupils and focus.

This morning in dull conditions I tried to find out the pinhole size that I use with an almost closed fist to bring distant objects into focus.

I used an accurate 1.0mm pinhole in stainless steel as my test pinhole. This was the smallest I had nearby although I may have smaller ones.

I estimate that a 0.7mm pinhole is what I was using with my fist.
This brought chimney pots at 120m and 124m into quite good focus.
Comparable to that using simple 2.25 dioptre glasses. I am far sighted and things are blurred with my normal vision without glasses.
These 2.25 dioptre glasses don't give the fine resolution I get from my prescription glasses, but the image is pretty good.

It is raining today, so I don't have bright sunshine.
But in bright light I think I use 0.5mm or 0.4mm pinholes for best images.

People here might complain about diffraction, too dull images etc. But this is what I actually use.

In addition, in bright light the best magnification for detail in my 90mm Maksutov is 125x or 0.72mm (0.7mm) exit pupil.

There are other cameras besides the Contax that use a movable film plane to focus.

As to how optical designers learn.
They used to go to the optical imaging department of Universities, but now they usually learn on the job. They have Mathematics degrees as a basis.

In the past people like Gauss, Fraunhofer, Newton, Schmidt, Baker, Vaisala or Dall just worked it out themselves.

Quite off topic, the new large liquid mirror telescope in northern India situated near the Himalayas at 8,500 ft uses thin Mylar just above the surface to stop air currents disturbing the rotating Mercury. So it seems this does not impact the image.

The proposed 20m to 100m Liquid mirror telescope at the Moon's north pole cannot used Mercury, as it would immediately evaporate there. Other liquids are contemplated.

B.
 
What I like to understand is that why a healthy human eye can not focus on individual objects located at various distances while looking through binoculars? I am not saying why those objects don't all appear sharp. I am asking why it is not possible to look at individual image points and see them in sharp focus?
Yes, Henry's post answered this question. The eye has only a limited range of accommodation, which is more than adequate for the entire range of distance with unaided vision (which after all it evolved for!) but cannot handle the proportionally greater amount of defocus that comes from typical magnifications of 8x or more. And this only gets worse with age.
 
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Yes, Henry's post answered this question. The eye has only a limited range of accommodation, which is more than adequate for the entire range of distance with unaided vision (which after all it evolved for!) but cannot handle the proportionally greater amount of defocus that comes from typical magnifications of 8x or more. And this only gets worse with age.

I am not convinced by the arguments presented so far. A binocular is an "afocal" teleconverter attached to the eye. If a healthy eye can focus from 20cm to infinity; the same eye with a 10X binocular in front of it should be able to focus from 2m to infinity. No focusing inside the binoculars should be necessary. The focused image may have a shallow depth of field due the combined f-number of the entire optical system but that's not my concern. The very ability to focus (or lack thereof) is what I am trying to understand.

In any case, let's not spend any more time on this question for now. If I find an explanation, I'll post back later on. ;) It is Friday and nice weather here in westside of Los Angeles. Life in this part of LA is all about the "image" you project and the shallower your "depth", the better:

https://youtu.be/-q-4XKTHJGs

B :)
 
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...Autofocus in cameras is now so precise, I think that autofocus binoculars should be looked at again...

The extraordinary autofocus of today's top-end cameras wouldn't be practical to implement in bins without bulking them up a lot and adding to their cost. Modern AF systems use a lot of data (including multiple focus/contrast sensors, color sensors, brightness distribution) with a lot of computing power to analyze the scene to pick an appropriate focus point. The best focus and metering systems are composition and subject matter "aware", including face "aware" and eyes "aware".

Simpler, would be to use an active system. Couple today's laser rangefinder technology to a power focus.

--AP
 
Omid, This depiction of L.A. is far more realistic...

https://www.youtube.com/watch?v=N-BmxK-0Jts&list=PLFv4y-PVQCXAzzp181XIut2hCBb5wGwIC

As for the focusing conundrums, Henry's explanation helps, but doesn't answer other questions, such as how the aperture/exit pupil size increases, or diminishes, depth of field. Also, how moving the focal plane towards the viewer at higher magnifications will eventually make it impossible for the eye to accommodate at any age. More offline reading to do...
Enjoy your weekend.

-Bill
 
If a healthy eye can focus from 20cm to infinity; the same eye with a 10X binocular in front of it should be able to focus from 2m to infinity.
Should? According to what? We don't get to make up the laws of optics, or facts of biology. But if you still want to try, you could invite Rico to enjoy the afternoon at your café, and see who can project an image farthest...
 
Should? According to what? We don't get to make up the laws of optics, or facts of biology.

According to the definition of an afocal system. An afocal system manipulates angles of rays by a simple multiplication factor M as I have shown in the attached drawing. If an input ray enters the entrance pupil of this system at an angle Alpha, it will come out of the exit pupil at an angle M x alpha. The output angle does not depend on the height (from the optical axis) at which the ray enters the system. (In a "focal system", the exit angle depends on both input angle and input height of the ray.)

I do know from experience that we can't focus on various objects seen through binoculars using the accommodation power of our eyes. Binoculars seem to provide only one plane of focus and the eye lens doesn't seem to be able to change this plane much. I am trying to understand exactly why and I want to be able to explain it using well-known laws of geometrical optics. I might be missing something which seems plain or obvious to you guys, so bear with me till I figure out why ;)

-Omid
 

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Omid,

There's another approach to thinking about this that might, or might not, be helpful.

Coupling the eye to a binocular or scope is not fundamentally different from using a camera for afocal photography through a binocular or scope. For our purposes the eye could be thought of as a camera equipped with a lens with a focal length of approximately 20mm. In daylight, with the eye's pupil closed to 2.5mm it would be the equivalent of a 20mm f/8 camera lens. When that lens is combined with a 10x binocular it's just as if its focal length has been increased 10x to 200mm and because the entrance pupil is now the 25mm central part of the binocular objective lens the focal ratio remains f/8. There are various online DOF calculators that can be used to compare the DOF at any distance of a 20mm f/8 lens and a 200mm f/8 lens.

Henry
 
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According to the definition of an afocal system. An afocal system manipulates angles of rays by a simple multiplication factor M as I have shown in the attached drawing.
Sure enough... so now instead of imagining how the eye "should" be able to focus anyway, you have to first envision what it means for an object to be out of focus (because something at another distance is in focus). The rays from this object are converging in front of or behind the retina instead of nicely on it, and the question is by how much, and whether the lens can distort enough to bring that focus onto the retina instead. Normally it can. (A diagram might help, like those at this DOF tutorial at Cambridge in Color.)

Now put a 10x binocular in front of the eye: all the angles get multiplied accordingly, so the difference between the angles subtended by the two objects also does. The distance by which that convergence misses the retina increases dramatically, along with the diameter of the "circle of confusion" for that object. Now it's way out of focus, not just more than before but more of a blur than anything encountered in unaided vision. Can the eye lens distort enough now to correct even for that? No, it can no longer shift focus to this object without adjusting other lens(es) in the system.
 
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Sure enough... so now instead of imagining how the eye "should" be able to focus anyway, you have to first envision what it means for an object to be out of focus (because something at another distance is in focus). The rays from this object are converging in front of or behind the retina instead of nicely on it, and the question is by how much, and whether the lens can distort enough to bring that focus onto the retina instead. Normally it can. (A diagram might help, like those at this DOF tutorial at Cambridge in Color.)

Now put a 10x binocular in front of the eye: all the angles get multiplied accordingly, so the difference between the angles subtended by the two objects also does. The distance by which that convergence misses the retina increases dramatically, along with the diameter of the "circle of confusion" for that object. Now it's way out of focus, not just more than before but more of a blur than anything encountered in unaided vision. Can the eye lens distort enough now to correct even for that? No, it can no longer shift focus to this object without adjusting other lens(es) in the system.

QED.


LEE
 
So where did Omid go? Did this explanation do the trick? At least I'm sure I understand this now myself. I hope it doesn't spoil patent #10 :)
 
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