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Binocular Evolution II: Curvature and Distortion (1 Viewer)

I can only guess it has something to do with the optical relationship between the eye's Petzval surface to the inner curvature of the retinal surface (which varies between individuals)

Hi Ed, interesting insight and hypothesis. I have simulated 4 combinations to look into this concept of field curvature to retina curvature matching. Bino is 60degree AFOV. The rays leaving the retina is just there to help you see where the focus was.

The simulation suggests that long eyeballs (myopia) is better matched to curved field binoculars, and emmetropia (no glasses) is better matched to flat field binoculars. This is of course looking at one parameter in isolation, and may not reflect actual population preferences.

1. Emmetropia (no glasses) + no field curvature
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2. Emmetropia + field curvature

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3. Myopia + no field curvature
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4. Myopia + field curvature
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I don't see where the field curvature of bins comes into those eyeball diagrams, and didn't follow the argument. And I'd still like to understand why some people perceive a flat field as, well, "flat" when I don't. Ed makes this sound rather like a duckrabbit, but I only see the duck.


But to refocus, my original questions can be answered to this extent:

1-2. Field curvature and distortion do turn out to be related in practice. Complex (mustache) distortion and "Absam rings" of softness are side-effects of aggressive field flattening, and have been managed better since introduction of the original EL SV. Low overall pincushioning may be necessary for an eyepiece to achieve a truly flat field, within other reasonable constraints.

3. Flat-field designs may be taking over the high-end market anyway because more buyers actually value edge sharpness than are bothered by perceived side-effects. Or because marketing has made alternatives seem inferior, even though some have useful sharpness nearly to the edge. (And alpha manufacturers today seem unwilling to produce much of a range of models given sales volume at that price level, so alternatives disappear despite still being preferred by some. Diminishing choice is my real complaint.)
 
If I dot out the focal plane of looking into a curved field binocular, this is what it looks like on an Emmetropic eyeball. In a longer oblong eyeball, as simulated earlier, this curvature may fit closer to the retina curvature - perhaps leading to a sensation of greater edge to edge sharpness.

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But any topic into eye health is a sensitive topic so I won't go any further unless requested.
 
The simulation suggests that long eyeballs (myopia) is better matched to curved field binoculars, and emmetropia (no glasses) is better matched to flat field binoculars. This is of course looking at one parameter in isolation, and may not reflect actual population preferences.
So, I'm an eyeglass-corrected short eyeball hyperopic who has strong "flat field" perceptions whether or not I use glasses. How does my type fit into the scheme? Ultimately, the "flat-field" perception requires an explanation although it would probably fall on deaf ears to anyone who doesn't perceive it in the first place. :cry:

Personally, I have no objection to tracking down any hypothesis to the bitter end. But, I would agree with tenex that the simulation diagrams are not intuitively obvious to me. In particular, what does a field flattener do to the retinal projection diagrammatically?
Ed makes this sound rather like a duckrabbit, but I only see the duck.
And I only see the rabbit. 😛

-----------------------------
I'll be gone for a day or so.
Ed
 
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Sounds like we may have inadvertently confused field curvature and image curvature. For clarity I will use focal plane curvature vs image distortion curvature in the future.

My eyeball simulations were to do with focal plane curvature, perhaps not relevant to your observation about "flat card effect".

Reading your description of "flat card effect", it sounds more relevant to image distortion curvature.

A flat card in front of you will indeed look flat because no matter how big it is, it cannot exceed 180degrees field of view. Same is not true for a curved objects, which if extended to infinity will circle you 360degrees and then into infinite revolutions.

A binocular image corrected fully to rectilinear will indeed look like "flat card".

Astro binoculars are more likely to have high pincushion and low focal plane curvature. Same with astro telescopes. On my f/9 refractor at a scenic lookout - astro eyepiece showed strong pincushion as you'd expect and low focal plane curvature.

Curved field + edge falloffFlat field + sharp nearly to edge
High pincushione.g. ultravid, noctivid (less so), SLCprobably big porros for astro (smaller true fov)
Low pincushiondoesn't exist?e.g. NL, SF (based on what i read)
 
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Kimmik,

I've concluded that we speak largely different languages. :)

Sounds like we may have inadvertently confused field curvature and image curvature. For clarity I will use focal plane curvature vs image distortion curvature in the future.
Field curvature refers to the curvature of an image surface, in particular, the surface defined by the infinite collection of points that are in focus.
I don't understand your distinction.

My eyeball simulations were to do with focal plane curvature, perhaps not relevant to your observation about "flat card effect".
Then why did you bring them up?

Reading your description of "flat card effect", it sounds more relevant to image distortion curvature.
If one hasn't experienced the perception (as many people have not) then it's hard to conceptualize/discuss what might be relevant. Have you?

A flat card in front of you will indeed look flat because no matter how big it is, it cannot exceed 180degrees field of view. Same is not true for a curved objects, which if extended to infinity will circle you 360degrees and then into infinite revolutions.
You must be joking. :giggle: Where to begin?

A binocular image corrected fully to rectilinear will indeed look like "flat card".
I can assure you that the particular binoculars I used with field flatteners were not absent distortion, — nor did they induce the globe effect in my case.

Astro binoculars are more likely to have high pincushion and low focal plane curvature. Same with astro telescopes. On my f/9 refractor at a scenic lookout - astro eyepiece showed strong pincushion as you'd expect and low focal plane curvature.
I'm not into astronomy, but field curvature and distortion are still different optical aberrations that are not necessarily correlated.

Finally, you haven't answered my question in post #44 "... I'm an eyeglass-corrected short eyeball hyperopic who has strong "flat field" perceptions whether or not I use glasses. How does my type fit into the scheme?"


Ed
 
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I suspect that kimmik hasn't thought through this "flat card" issue and is speaking off the cuff. Frankly it confuses me too, because that metaphor suggests looking at an image rather than a scene, as through binoculars. Which of course is exactly how Ed is saying it feels... but why? Or rather, why would conventional bins not give him this impression? In both cases we have an artificially cropped view, so is edge softness somehow diminishing that effect in some way that matters to a hyperopic?
 
Kimmik,

I've concluded that we speak completely different languages. :)


Field curvature refers to the curvature of an image surface, in particular, the surface defined by the infinite collection of points that are in focus.
I don't understand your distinction.


Then why did you bring them up?


If one hasn't experienced the perception (as many people have not) then it's hard to conceptualize/discuss what might be relevant. Have you?


You must be joking. :giggle: Where to begin?


I can assure you that the particular binoculars I used with field flatteners were not absent distortion, — nor did they induce the globe effect in my case.


I'm not into astronomy, but field curvature and distortion are still different optical aberrations that are not necessarily correlated.

Finally, you haven't answered my question in post #44 "... I'm an eyeglass-corrected short eyeball hyperopic who has strong "flat field" perceptions whether or not I use glasses. How does my type fit into the scheme?"


Ed
 

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There is an explanation of the "flat card" effect in Holger Merlitz' bino book (in German it's called "Kulisseneffekt" - "Kulisse" is the scenery on a theater stage that consists of flat objects). It's caused by the magnification of the binoculars. Larger magnification makes it more obvious. All the distances between objects are optically reduced by the magnification factor. So things look unnaturally flat because our eyes are not used to see 3D at a distance. The effect is relative to the "plasticity" of the image so it is amplified in porro binos because of their "3D effect". A flat field design would make the effect more obvious, too but it's not caused by it if I understand the explanation correctly.
 
There is an explanation of the "flat card" effect in Holger Merlitz' bino book (in German it's called "Kulisseneffekt" - "Kulisse" is the scenery on a theater stage that consists of flat objects). It's caused by the magnification of the binoculars. Larger magnification makes it more obvious. All the distances between objects are optically reduced by the magnification factor. So things look unnaturally flat because our eyes are not used to see 3D at a distance. The effect is relative to the "plasticity" of the image so it is amplified in porro binos because of their "3D effect". A flat field design would make the effect more obvious, too but it's not caused by it if I understand the explanation correctly.
Yes, I believe Holger is correct. The perception is definitely related to the perceptual effect of axial magnification, which increases with M^2. However, the question is how does a flat field design make the effect more obvious? And if it does make the effect more obvious, is that not just another way of saying that it causes it?

I like Holger's word selection. "Kulisseneffekt" translates to "backdrop effect" in English, and a theater backdrop is literally "a [flat] painted cloth hung at the back of a theater stage as part of the scenery." I have every reason to think we're trying to describe the same perceptual phenomenon, but with no psychophysical theory to explain it.

Flacher Karteneffekt sounds very authentic too. :giggle:

Thanks,
Ed

* PS. I don't understand the meaning of "plasticity" of the image, much less how it relates to 3D perception or Flacher Karteneffekt.
 
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Yes, I believe Holger is correct. The perception is definitely related to the perceptual effect of axial magnification, which increases with M^2. However, the question is how does a flat field design make the effect more obvious? And if it does make the effect more obvious, is that not just another way of saying that it causes it?
If the basic perceived scale disparity that is observed in a magnified field of view is the primary effect being discussed, note that it can be present in unmagnified fields as well, but only comprises a portion of the image. How does a 'flat field' make it more obvious? Perhaps simply by showing more of the field in focus, and removing a false depth of field, out of focus, effect around the edge, which curvature of field provides/implies.

Yes, a narrow, distant field of view has a certain quality, but its 'natural' in a sense. Its just how scale relationships work when they are far from the viewer's position.
 
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I consider it fascinating how much are people bothered by field distortion - since it doesn't really bother me at all. I think it's all in how I came into using all sorts of telescopes through astronomy, where it's largely irrelevant since there are no lines to follow - how do you know your star cluster or nebula is distorted? Yes it's still noticeable when panning though star fields, but who cares, you are scanning the sky looking for moving subjects, you have to stop to see yhr faint things.

I think that many of you would be slightly appalled at the view through my Meopta 82 with my astro eyepieces, because the distortion is really obvious - but I really like the tradeoff in the astronomical direction: they eyepieces have a lot of pincushion, but they have 82 degrees AFOV, really sharp to the edges - unlike some of the scopes I have look through with original eyepieces where the tradeoff seems to be the inverse.

My thinking here is that I already know how a straight line looks like, so I am not terribly affected by not seeing those properly, but I am interested in the fine details! But I do understand that for many this is far more important as panning with a distorted field must be unpleasant even nauseating for many - I have simply the advantage that my stomach is disconnected somehow from my senses, I don't get seasick ever for example.
 
Perhaps simply by showing more of the field in focus, and removing a false depth of field, out of focus, effect around the edge, which curvature of field provides/implies.
I don't find that an out-of-focus edge provides a sense of depth, or understand how it would, or what any of this has to do with "depth of field". (A flat-field binocular will still show fore/background objects out of focus.)
 
I don't find that an out-of-focus edge provides a sense of depth, or understand how it would, or what any of this has to do with "depth of field". (A flat-field binocular will still show fore/background objects out of focus.)
It doesn't do it for me either, but I recall reading a description of curvature of field that suggested such a thing... that it implied a fg slightly out of focus, for example. I think any such illusion, if it did work, would be entirely dependent on the context of the image.
 
In the image of our binoculars we can observe all kinds of optical illusions:
The illusion of increasing the general contrast in low magnification binoculars.
The illusion of increasing brightness by tilting the light curve towards a certain wavelength colour.
The illusion of the globe effect when panning to some binoculars by managing geometric deformations.
The illusion of increasing the general contrast by exacerbating of yellow and green colours.
The illusion of increased clarity in binoculars with a greater depth of field.
The illusion of higher power than in reality, sometimes in low AFOV binoculars, when we compare them with ones with a wider AFOV.
The illusion of a 3d effect by managing geometric deformations on the edges of the FOV.
We also have other kinds of illusions. For example, the illusion of weight related to the volume distribution of weight. And a lot of other illusions.

But we notice one very important thing: that not all of us respond with the same sensitivity to these optical illusions, each having different visual abilities.
 
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To answer a much earlier question about the eyeball simulations:

Hyperopic short eyeball, the petzval curvature is better matched to field flattened optics.

Emphasise this does not take into account distortion difference/preference.

Ps dorubird, some of those effects are definitely real but I agree with the observations!
 
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To answer a much earlier question about the eyeball simulations:

Hyperopic short eyeball, the petzval curvature is better matched to field flattened optics.

Emphasise this does not take into account distortion difference/preference.

Ps dorubird, some of those effects are definitely real but I agree with the observations!
I agree that 'effects' is probably a better word here than 'illusion'.

Lee
 
This was an interesting read, giving me new terminology to work with.
EDIT - I'm referring in particular to a footnoted reference in one of the articles someone linked:
Distortion of the visual field
Holger Merlitz

How would one design a bin for a particular critical usage - for example, the WWII Navy binoculars based on the B&L 7x50 design. Two uses, and optimizing one might reduce the other ?
1) in an empty sky, discern pinpoint objects (enemy airplanes).
2) on the ocean horizon, discern objects (ships).

The SARD Mark 41 fetches high prices for its wide field even though it exhibits Rolling Ball effects. Its purpose is for an aerial observer to see wakes of moving ships.

For both needs, a wide field may be a good thing even if Rolling Ball exists. In an empty sky, Rolling Ball would not be discerned, but one would have to hold the bins stationary to determine the true direction of motion of the airplane.

On the horizon, could you minimize the "seasick" effect by installing a reticle line that follows the curvature of the Ball distortion line. You could even blackout the lens below the "hill." Then the brain would expect objects that are moving from the side towards the other side to "climb the hill." You might place the reticle so the high point is slightly below the center of the optics, to avoid it blocking the view of the horizon; and train the user to place the reticle line just below the horizon.
 
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Should I open another thread for my other question?
How do the distortions discussed describe what happens to the view when the users actual IPD is less than the minimum closure of the hinge?
My IPD is 53mm (under the 5th percentile by one chart I saw).
Most bins only go down to 55mm or 56mm.
In fact, I purchased Zeiss Conquest HD 10x42 and 8x32 because they seem to be the only brand at that quality level (Beta?) whose stated IPD goes down to 54mm.

I can choose to either center one eye, which becomes dominant in my viewing, and maybe negates much of the advantages of using a BI-nocular; or try to equalize the off-centering (which seems to be harder to utilize without eyestrain & vignetting).

I can also back my eyes away further, which reduces my FOV but seems to help when at close focus (my birdfeeder is ~20-25 feet away from the window). Which is odd, since isn't the binocular doing the all xxx accomodation xxx EDIT - I meant to say CONVERGENCE - at any focal distance? Should not my pupils be at "infinity" gazing into the ocular, no matter what the binocular focal distance is ? Does having a too-small IPD cause at least one eye to converge & suffer eyestrain?

EDIT: my opthalmologist & his glasses fitter were unable to answer my question "for binoculars, are my pupils at a distance of Close IPD, Intermediate, or Far." * The fitter measured me at her arm's distance, for "office" strength Transitions lenses. I am slightly nearsighted. She said the rule of thumb is to add 1.5mm per eye to determine Far IPD.
* American and British usage of these terms differs.

EDIT - getting more terminology from a previous topic, esp. re some effects Porros have at close range.

see birdforum.net/threads/is-3-d-more-natural-in-porros-or-roofs.425235/page-2

Omid Post# 24
...The exact technical answer to your question is that both models provide diminished stereoscopic effect for far objects. In other words, binoculars deliver L and R images whose stereoscopic parallax does not match the dioptric distance of the images seen through the eyepieces. As a result, binoculars create vergence-accommodation conflict in the human eye.

L-R Parallax: amplified M-times and further multiplied by the ratio of objective IPD to eyepiece IPD if applicable
Accommodation demand: amplified MxM-times then a constant value (depending on focus knob position) is added to it or subtracted from it.

Resolving the conflict between accommodation (eye focus) and parallax (eye convergence) requires that the objectives are separated M-times larger than the observer's IPD. A stereoscopic viewing condition in which parallax and focus are consistent is called "Ortho-stereoscopy". It is discussed very rarely in texts. There is a brief review in Le Grand (Form and Space Vision, 1967) on page 312 just before he ends his book on page 313. The formula there is consistent with my own calculation that ortho-stereoscopy in binoculars requires M-times larger IPD on the objective end. For a 10X binocular, this require approximately 65cm separation between the objectives (obviously not practical).
 
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