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New Horizons II (1 Viewer)

Tringa45

Well-known member
Europe
Omid,

Henessey is or was a professor of psychology, a diffuse "science" to which I have a particular aversion (I was once married to one).
I did not understand a word of what he wrote and suggest that his understanding of telescope optics is similarly lacking together with that of Home and Poole.
Holger Merlitz' book, "Handferngläser" (Hand-held Binoculars) can be strongly recommended but, I think is only available in German.
"Telescope Optics" by Rutten & van Venrooij is something of a standard work.
If one has understood the basic content of these, then one is less vulnerable to such flat-earth theories as "Instrument Myopia."

John
 
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tenex

reality-based
Antarctica
Omid's presentation of this material has not been organized, and I have the impression that talk of virtual images etc is needlessly complicating the subject of viewing through an optical instrument. On the other hand, I gather that there is disagreement (hard to believe in 2020) about the physiological resting focus of the eye itself, the classic view being infinity, while Omid cites studies of dark/empty focus suggesting a meter or two instead, which actually seems more plausible to me. (I doubt that the eye could be so constructed as to be perfectly focused at infinity when relaxed. Is this really what Helmholtz suggested?) I need a clear overview of the subject, and really don't want to discuss or argue these questions further with so much uncertainty.
 

Binastro

Well-known member
Camera viewfinders are set by convention at minus 1 dioptre.

I always assumed that good quality binocular zero settings are at infinity.

Camera lenses have an infinity setting, with an infra red mark, which is different for most lenses.

Regards,
B.
 

Omid

Well-known member
United States
Resting focus state of the eye - final part

I'd like to conclude my discussion of "resting state of accommodation" by posting a 1984 paper Prof. Fred Owens which provides a readable review of this subject for non specialists. Fig. 4 in this paper illustrates how, as the quality of focusing stimulus degrades, the human eye becomes less responsive and eventually stays at a constant focus state:

Dark_Focus.jpg

The focus stimulus-response plots in this figure are symbolic (i.e. show a simplified representation of actual scientific measurements) but very illuminating nonetheless. Note also that the horizontal axis represents decreasing distance, and is logarithmic in cm units. This gives a uniform increasing scale for accommodation demand in diopter units.

I trust that forum members who are interested in learning more about human vision will enjoy reading this paper and learning from it as I did.

-Omid

PS (@ Binastro): Thank you for mentioning the diopter setting of camera view finders. I have also read studies by the Canadian military researchers who recommended -0.5D to -1D setting for the eyepiece of certain night-vision devices. Such anecdotes are consistent with the "intermediate resting state" a.k.a. the "instrument myopia" theory.

Now, do you know any optics books or official manufacturer brochures that positively confirm our assumption about the meaning of the zero diopter setting on individual-focus binocular eyepieces?

PPS(@ Tenex and John): Thank you for following my discussion of this subject. It is perfectly OK to keep a skeptical attitude towards this theory. I myself didn't know about any of these studies until about a year ago. They show how complex and fascinating we human beings are!
 

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tenex

reality-based
Antarctica
This Owens paper is very interesting, nice to read consecutively instead of merely encountering disjointed bits in Omid's earlier posts. The argument seems plausible, and agrees with my own experience: by the method Omid previously suggested, I measured my own resting focus as about 2m (-0.5 D), which is on the long/low side of the range revealed by experiment. Incidentally this suggests relatively sharp night vision, which I've always felt I have.

"Instrument myopia" however appears to be entirely fictitious, problematic only on the false (classical) assumption of resting focus at infinity. This paper doesn't address it, presumably because it's simply been explained away: the way people tend to focus optics is now the natural expectation and perfectly comfortable, instead of a problem that would cause eyestrain. Fascinating, but I still don't see how it actually matters for instrument design, as Omid suggests:
All hand-held binoculars are optimized for use by a non-existing creature whose eyes have parallel lines of sight and a natural focus point located at infinity.
How so? Helmholtz's error has not been somehow built into binoculars. Their range of focus seems perfectly adequate for use by existing humans, no matter what their individual resting focus.

[Edit: disregard the third point in my post #217, which is irrelevant now that I've read what the experiment generating the graph actually was -- not involving binoculars. This kind of confusion wastes time and effort.]
 
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Omid

Well-known member
United States
Fascinating, but I still don't see how it actually matters for instrument design, as Omid suggests.

I am very pleased that you are now a believer in these new theories of human vision! Welcome aboard! B :)

I'm afraid I can not discuss my binocular design concepts in this lovely avian peek-a-boo forum. The proper place to look for such information is the United States patent application data base and for that you have to wait 18 to 36 months ;) In a few days, I will start a new topic: stimuli for accommodation. Accommodation is the ability of the human eye to change its focus from distant to near objects (and vice versa). This process is achieved by the lens changing its shape. But what exactly are the cues to accommodation? How on earth does the brain determine if the image on the retina is "in focus" or "out of focus"?! If the retinal image is out of focus, how does the feedback system that controls accommodation know in which direction (far or near) it has to adjust?!

stay tuned..

-Omid
 
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tenex

reality-based
Antarctica
"New theories"? Forty years ago. (About time to come aboard. ;))

"I will start a new topic" -- A modest suggestion: just post a reading list instead, to inspire genuine discussion. The way you've been dispensing bits of information(?) in cute little questions and answers is too annoying for me to follow further.
 

Omid

Well-known member
United States
Whether by intelligent design or by evolution, the idea of a perfect aberration-free eye, like a Leica or a Canon camera, forming a perfect image of the world on the retina is appealing. Here, I present an interesting exchange of ideas between two leading researchers in the field of physiological optics. In a first paper (attached as PDF file), Professor Rafael Navarro (a world-renowned Spanish researcher) wonders why the eye has a wide-angle lens but a very narrow sharp field of view:

A most intriguing design flaw is the contradictory optical and retinal designs. There is no doubt that the eye is a very wide-angle lens and its design seems to aim to guarantee a high homogeneity of optical quality across its wide visual field, whereas the retina is highly inhomogeneous; i.e. the vision of details is concentrated in a small central area, the fovea, and is rapidly lost as we move towards the periphery. This severe mismatch between optical and retinal resolution (possible design flaw) can only be explained by evolution.

In the second PDF file, Prof. P. Kruger (another well-known scholar, now retired) responds to the above "design contradiction" by explaining how the crude flaws of the eye morph into ecologically useful features if we don't consider the eye as a camera and instead follow Gibson's theory which considers the eye as an "ecological sensory organ":

James Gibson … insists that the optics of the eye and the resulting retinal image are irrelevant for ecological optics. In Gibson’s view, information from the environment is structured into the optic array, and the eye simply “picks up” invariant relationships in the array that specify the environment. In this model the “perfect” eye all but disappears!

Enjoy the papers :)

Note by Omid: Prof. Kruger's own research have confirmed a unique feature of the human eye (originally noticed by the British researcher E. F. Fincham in 1951) : The eye is capable of sensing "wavefront error" which is why it can accommodate in the correct direction nearly all the time. Unlike the autofocus system of SLR cameras, the human accommodation system never stutters. However, the neurological control signal which is used to drive the eye's accommodation reflex is not available to conscious use. A human operator acting consciously can't focus an instrument (such as binoculars) in the right direction solely based on the defocused image that he sees. He has to make a 50/50 guess as to which direction he should turn the focus knob.
 

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Omid

Well-known member
United States
Entering the Dark Zone

The conventional wisdom holds that human eye pupil dilates to a maximum of about 7mm under low-light conditions. Therefore, binoculars that offer an exit pupil of 7mm or more (such as 7X50 or 8X56) are particularly suited for low-light and night viewing.

Let's consider viewing a natural scene (such as an owl sitting on a distant tree) under low-light conditions. This means, we are not discussing using binoculars for viewing stars or moon which are self-luminous high-contrast objects. We are dealing exclusively with observing a low-contrast natural outdoor scene at night. During the night, the human eye switches to the "scotopic mode" of vision where rods in the retina are the functional photo receptors. This means:
  • No color vision (color vision is provided by cone receptors which are not functional at night)
  • No foveal vision (there are no rods in the fovea which is a small part of retina responsible for the high-resolution central portion of our field of view)
I am not concerned with seeing color but lack of foveal vision at night has very significant consequences when using binoculars: The very center of FOV is a blind spot! When a human being "looks" at a precise direction at night, he sees nothing in that direction. His brain fills in this "central hole" using the information from the surrounding retinal regions (which do contain rods). That is, the visual system infers the content that might be present in our direction of gaze by interpolation not by actually seeing anything there .

So, if I look directly at the owl on the tree using my outstanding Zeiss 7X45 "Night Owl" binoculars, I won't see it. I might see "an owl" if I deliberately deviate my line of sight a few degrees so that the owl will move into my peripheral vision. Even then, what I might see will be a vague form without any details (the eye's acuity diminishes significantly under scotopic viewing conditions, many rods are connected to a single neuron that connects them to the brain. This increases light-gathering power of the eye at night at the expense of acuity).

Another tiny insignificant fact which manufacturers never advertise is that you can't focus your binoculars at night. You keep playing with the focus knob but that only turns one vague form into another. Focusing requires foveal vision which is not available at night. Night vision, for human beings, is primarily a proprioceptive system whose function is guiding body posture and locomotion. It is not designed to "focus" on anything.
 
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Omid

Well-known member
United States
Entering the Dark Zone - Part II

In this post I explain why very large exit pupils (more than 5mm) and very high light transmission (say more than 85% ) have very minimal affect on the performance of binoculars in twilight. Again, I emphasize that my analysis is confined to viewing nature scenes as encountered during hunting or bird watching. This analysis is not related to viewing high-contrast self-luminous point-source objects such as those encountered in astronomy.


Fact I - Human eye does not perceive brightness proportional to the area of the eye pupil. This is due to an effect known as StilesCrawford effect of the first kind. The graph below shows the relative sensitivity of the cone receptors of human eye as the pencil of light entering the pupil gets larger. As you can see, the light that enters near the edges of eye pupil is much less effective in creating "perceived brightness".

1608401430575.png

Fact - II: Human eye is not designed to respond to brightness. Brightness is not a characteristic of objects that we encounter in nature. Oranges, flowers and snakes don't have a characteristic brightness. They have a characteristic reflectance that defines their apparent color and texture. Our eye automatically perceives this reflectance and ignores brightness changes. According to Weber's law, human perceptual threshold for detecting a change in brightness is about 8%. This means, on average, people can not perceive a change in brightness less than 8% of a base level. This law implies that an increase of less than 8% in light transmission would be barely perceptible to most people, even those with young healthy eyes. Again, this is not the eye's fault. It is exactly what human eye is designed to do: ignore brightness (a highly variable quantity) and perceive reflectance (a stable feature of objects and environment).

Fact III: Even if your eye pupil enlarge to 7mm or more, this may not help with resolving details in twilight. A wide open pupil increases the eye's intrinsic aberrations so visual acuity decreases. Human eye achieves maximum visual acuity at pupil dimeters about 2-4mm (Campbell and Gregory, Nature, Sept. 24, 1960, page 1122). At low light, the eye's acuity is very poor and it is independent of pupil dimeter (if artificially constricted).

Fact -IV: Magnification can help the eye see better in twilight by enlarging the object such that it's spatial frequencies fall within the mid-level range of frequencies where the eye can still perceive and resolve details. That's why a 10X50 could be better than a 7X50 in some cases. Picture below shows how the eye's contrast sensitivity changes with spatial frequency. Spatial frequency is measured as number of black and white stripes (cycles) per visual angle (degree). Looking at a low contrast target, the eye can best discriminate spatial detail in the range of about 1 to 5 cycles per degree. Looking at high-contrast targets, the eye can resolve up to 60 cycles/degree.



1608401319591.png


Finally, the most serious obstacle in using binoculars under low light is "focusing":

Fact -V: As luminance fades, so does contrast of objects (animals, birds, etc.) against their background. Under these conditions, human eye loses a significant portion of its accommodation power. At very low luminance, the eye simply ceases to accommodate and stays at a constant focus state known as "dark focus" state. At the same time, the retina does not provide sufficient blur feedback to allow manual focusing of the binoculars by turning the focus wheel. Lack of precise focus causes more blur in the image which in turn causes lower perceived contrast, a vicious cycle.



1608402633026.png


Fact - VI: The response time of the retina slows down (signals are sent to brain with more delay) when luminance decreases. The delay in providing visual response to the brain causes the hunter's fingers to overshoot or undershoot the binocular's focus adjustment. (In technical terms, the feedback loop oscillates instead of settling on a fixed value. This is similar to our experience when taking a shower: It is difficult to adjust the hot/cold mixing valve due to the time lag between the time you adjust the valve and the time water pours down from the shower head) .

As a result of factors V and VI mentioned above, a hunter or bird watcher will find it nearly impossible to focus his binoculars in twilight.
 
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Sterngucker

RⒶdneck
United States
My resting focus is at ca. 1.3 m as measured per the Omid method. That would be 0.77 D, correct?
 
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Omid

Well-known member
United States
H
My resting focus is at ca. 1.3 m as measured per the Omid method. That would be 0.77 D, correct?

Hello Sterngucker,

Yes, 1.3m corresponds to 0.77D which seems a reasonable value for your eye's "resting focus position". Did you measure it using a manual-focus SLR camera as I suggested in Post #211?

A few months ago, I measured my own resting focus distance using riflescopes: I focused the eyepiece so that the reticle looked sharp in a few riflescopes. Then used a Nikon SLR camera equipped with a macro lens to see how far the reticle's virtual image is. It turned out that I focus the eyepiece so that the reticle image is somewhere between 65-75cm away from my eye position. So, my own resting focus is about 1.4D.

I have been searching for other methods of measuring focus state of our eyes while using binoculars and riflescopes. One other method which seems somewhat promising is to use the strong coupling between eye convergence and eye accommodation: If we look through an instrument with one eye (preferably our dominants eye), then the other eye will both accommodate and converge so that its direction of gaze fixates on the accommodation target of the dominant eye. Again, by doing some simple tests using riflescopes, I found that my left eye's line of sight indeed converges towards my right eye when looking through a riflescope. The amount of convergence corresponds to fixating on a point 60-80cm away which further supports the notion that the virtual image I am observing inside the riflescope is somewhere within this range.

It would be interesting if we could test the change in our state of accommodation when our mental and emotional state changes as predicated by the modern findings that say the resting state of accommodation is determined by the balancing action of the sympathetic nervous system and the parasympathetic nerves system (See post #216). Do we focus our binoculars the same when we are calm vs when we are frightened? Could differences between your resting state of accommodation and mine be associated with our temperament traits and personality? These are interesting questions. The answers might not change the way we use binoculars, but they could teach us something new about our own amazing sense of vision and its deep connections to our inner self.

-Omid
 
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Sterngucker

RⒶdneck
United States
H


Hello Sterngucker,

Yes, 1.3m corresponds to 0.77D which seems a reasonable value for your eye's "resting focus position". Did you measure it using a manual-focus SLR camera as I suggested in Post #211?

...

-Omid

I did indeed. The approximate value is due to the fact that my old Canon FD 50 f/1.8 has markers at 1.2 m and 1.5 m so my 1.3 m is a measured guestimate. Good enough for me anyway. Same as my measurement of my dark-adapted pupil 6 mm.
 

Omid

Well-known member
United States
Entering the Dark Zone, Part III

In this post I elaborate the distinction between seeing at night (scotopic vision) and seeing during twilight (mesopic vision) further.


Mesopic_Vision.jpg

The human retina contains two main groups of light receptors: cone receptors and rod receptors. (Each of these categories have their own sub categories). These receptors are shown in the diagram below:


Eye_Diagram.png

Night or "scotopic vision" is mediated by rod receptors in the retina. This mode of vision is primarily a "surround vision" sense. It provides us with the extremely important - to our hunter ancestors not to us Amazon Prime shoppers- ability to maintain an upright body posture and walk in the dark. Surround vision helps the brain pick up the horizon line and other significant features of the terrain that are necessary for proprioception which is the sense of self-movement and body position.

The diagram below -from the website Amateur Telescope Optics- shows how rod and cone receptors are distributed across the horizontal field of view of the eye. Note how it shows that there are no rods in the fovea which is the small central part of the retina that defines our "line of sight". This area has cone receptors only. Due to this unique distribution of cones and rods, under scotopic conditions the ability to focus on an object using frontal vision is lost but the ability to maintain posture and walk in the dark is maintained. Under scotopic conditions, we are literally blind in the center of our gaze. Our visual system fills this gap with information from the surrounding retinal regions (which do contain rods) so we "think" we see things at the center of our vision while, in truth, we don't. Stated simply, under scotopic conditions we can still "see our environment" but we cannot "look at specific objects".


Eye_Diagram_02.png


During twilight vision, both rods and cones are active. However, our ability to look at things is still mediated by cones. This is because when we "look at something", we use foveal vision which is mediated by cones. When we see color, such as when seeing colorful stars at night, this too indicates a vision mediated by the cones.

Therefore, when we pick up our binoculars to look at an owl sitting on a three in twilight, our vision is primarily mediated by the cone receptors and our perceived brightness is affected by the Stiles-Crawford Effect as discussed in Post #230 above. The simple example below shows how increasing a binocular's aperture will not lead to a proportional gain in the light energy absorbed by the cone receptors:

Exit Pupil Diameter IncreaseBeam Energy Increase (geometric, based on beam area)Effective Beam Energy Increase (Stiles-Crawford Effect considered)
From 5mm to 7mmabout 100%about 40% (rough estimate from the curve)

The large exit pupil of binoculars such as Zeiss 8X56 Night Owl (or Swarovski SLC 8X56 latest generation) provide advantages other than increased brightness which makes them effective for hunting or birdwatching at twilight. The large exit pupil of these binoculars when combined with long eye relief and relatively low power provide unique visual advantages which will be discussed in a future post.

Sincerely,
-Omid
 
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Sterngucker

RⒶdneck
United States
Is this rod/cone distribution the answer to Troubador's question/observation regarding 'averted vision' in the dark/dawn in The Other Thread?
 

Omid

Well-known member
United States
Is this rod/cone distribution the answer to Troubador's question/observation regarding 'averted vision' in the dark/dawn in The Other Thread?

Yes. The "averted vision" technique takes advantage of the fact that rod receptor concentration - which mediate vision at very low light- peaks a few degrees away from the natural direction of gaze of the eye. So, if you intentionally look to the side of an object under very dark conditions, you may be able to see it. But this is a very unnatural method of seeing and we humans are not conditioned to look at things this way. See this recent paper by researchers at University of Marburg in Germany.


Rod_Cone.jpg


Two other interesting things to note:

a) Many rods (about 100) are connected to a single ganglion cell which then connects to the brain. So, the light arriving at many rods add up to form a single "large pixel" so to speak.

b) Rods have a slow communication channel to the brain. So, it is likely that rod ganglion cells integrate light energy over time as well.

Combination of (a) and (b) creates a condition similar to a film camera with a coarse-grain negative and slow shutter speed. Light sensitivity is increased by integration over both time and space. The price paid for this increased sensitivity is poor spatial resolution and slow reaction time.
 
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Sterngucker

RⒶdneck
United States
Looking at that diagram reminds me of a question that has been bugging me for ages.
Do you have any explanation for the existence, and indeed location, of the blind spot?
I mean, if there was some sort of need for it to exist, placement a bit more out of the way as it were would be more logical. Not saying that the logic of evolution is always obvious to the untrained mind.
 
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Binastro

Well-known member
I confirm that there is an integration time involved for seeing the very faintest stars.
It is thought to be up to six seconds.
It involves skilled averted vision.

In addition, the photons arrive at intervals.

I think that two photons are needed, but possibly only one.

I take a faint star as seen with three views in the same place.
But if possible I wait for 4 to 6 views.
Some observers wait for up to twenty minutes to record the very faintest stars.

Random single flashes are not stars.

Observations of extended faint sources are different but still need averted vision.
With a flying dimly lit owl or buzzard the angular speed is important, and less predictable.
I have followed high flying dimly lit probable buzzards in the Canon 18x50 near the limit of vision.

I was very much better at these types of observations up to age 50.

Regards,
B
 

Omid

Well-known member
United States
@Binastro: Thank you for sharing your experience confirming that human eye does indeed integrate light energy over time to enhance sensitivity under scotopic conditions. Very interesting observations!

@Sterngucker: Your question can be addressed as part of a more fundamental question: Why is the human retina wired inside-out?!!!! We have a blind spot because the human retina - like that of all vertebrates- is wired inside out! The nerve "wires" and other neural aparatus that connect the phtoreceptors to the brain are positioned on top of the phoro receptors -on the lens side- not behind it! This means, focused light from the lens has to pass through several inner layers of neural apparatus and capillary blood vessels before reaching the photo-receptor layer:

scars_eye_II.jpg

scars_eye.jpg

Some noted scholars such as Profs. Trevor Lamb and Daniel Dennett consider this a design flaw and an "evolutionary scar". The above diagram is from an article by Prof. Lamb published in Scientific American.

The retina layout is certainly very strange, but I don't agree with the view that this constitutes a flaw. In what exact way is our species handicapped by a defect in our vision? Human vision is above and beyond miraculous in terms of range of its adaptation and capabilities. We don't need more visual resolution to survive or function. In fact, our brain discards a huge amount of visual information in the process of selective attention. The eye is not a camcorder as Prof. Bennett thinks. Our eyes do not "take pictures", our brain does not see an "image" of the world. Why human retina is inside-out remains an unsolved mystery.
 
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Sterngucker

RⒶdneck
United States
...

@Sterngucker: Your question can be addressed as part of a more fundamental question: Why is the human retina wired inside-out?!!!! That is, why light has to pass through several inner layers of neural apparatus and capillary blood vessels before reaching the photo-receptor layer? You can see this in the diagram in post #234. Some scholars consider this a design flaw and an evolutionary scar. The retina layout is certainly very strange, but I think there must be a functional explanation for it. The eye is not a camera, it does not "take pictures", our brain does not see "an image of the world".


View attachment 1360812

Hmmm ... this is a bit like being back at university ... every answer I got from a lecture or personal turorial threw up new questions. This thread is rather rejuvenating in this respect.
I can relate to the idea of the eye not being a camera in the way we understand the camera obscura (or its modern equivalents), but the idea that the brain does not see, or maybe form, an image of the world is rather alien and leaves me baffled but I assume you are referring to Helmholtz's unconscious interference.
 
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