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Perceived field of view

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Old Monday 2nd September 2019, 22:22   #1
tenex
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Perceived field of view

This subject has come up in a couple of recent threads and seems worth asking about more directly: I don't use eyeglasses with binos, and have the impression that what I'll call my perceived field of view (PFOV) -- how immersive the view feels, how much I feel I can see vs the black space around it -- suffers from two recent trends, high eye relief and field flatteners. And yet neither one is a factor in the equation that defines "Apparent FOV", which is only a function of real field and magnification. That has tended to confuse discussion of the issue for me.

In the case of FF, I think I can envision how this works: a curved field seems a bit bigger subjectively because things get stretched at the edges, while a flat field irons that out. I presume this is why I've read that the ISO formula for AFOV, which gives a slightly smaller result, is more accurate for FF binos while the simpler approximation has always been good for classic designs.

Regarding ER, I have the impression that among binos I've tried, those with around 13mm ER or less (maybe up to 15mm or so) give me a more immersive view, while ER approaching 20mm as in many models today gives more of a tunnel-vision effect. Ideally I mean this other things (magnification, RFOV) being equal, though in practice that probably hasn't quite been the case. But again it makes sense to me that you can't increase ER indefinitely while maintaining an immersive view.

Do I have this about right, and can anyone offer further technical explanation? Or is it the case that high-ER designs actually wind up limiting the real FOV, and thus the apparent as well?

Last edited by tenex : Tuesday 3rd September 2019 at 03:49.
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Old Tuesday 3rd September 2019, 02:53   #2
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Well, that is interesting at least for 10X50s
Nikon WX 10X50 - ER -15.3 MM, 89 degrees AFOV, RFOV 8.9 degrees
Leica 10X50 - ER - 15MM, 66 degrees AFOV, RFOV 6.6
Swarovski 10X50 ELSV - ER - 20MM (listed by Allbinos), I think it is a bit less according to Roger Vine ~ 15.5 MM) 66 degrees AFOV, RFOV 6.6
Swarovski 10X56 SLC - ER - 19.5 MM, 60 degrees AFOV, 6.3 RFOV

The Zeiss SF 10X42 seems to have almost 18 MM ER and 6.8 degrees RFOV, the Monarch HG 10X42 has almost the same ER and slightly more FOV, so it looks like these 10X42s have stretched the envelope. The SF is immersive I would think, seemed that way to me.
I am sure I am missing some particular model, but there are not that many binos that will give a larger than normal FOV with ER approaching 20MM

One thing for sure, it sure would be nice to have the WX 10X50 specs in the size of the other glass listed.

Andy W.
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Old Tuesday 3rd September 2019, 11:23   #3
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Using a (big) bino which can take Astro eyepieces will help. For instance the Morpheus eyepieces have 76 degree apparent field of view and advertised 20mm eye relief and are well regarded. There are others with wider fields still. Once you try using well corrected ultrawide eyepieces (>70degree) going back to a pair of normal binoculars (like the Nikon 8x30 E2) feels like a claustrophobic disappointment.

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Old Tuesday 3rd September 2019, 15:20   #4
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Field curvature is I think different to distortion in this context, which seems to be the problem with the original post.
Field flatteners correct curvature but I don't know if in practice they affect distortion also.

Much of the difference is probably how our brain interprets the visual view.

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Old Tuesday 3rd September 2019, 15:58   #5
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I agree with Binastro. Eyepieces can be designed to combine low field curvature with any desired distortion or combination of distortions. In the binocular world the false notion that flat fields are always combined with low pincushion distortion seems to have originated because that's the combination that Swarovski happened to choose for the field edge in the Swarovisions. The distortion choice affects the ratio between real and apparent field by either compressing objects near the field edge (barrel or low pincushion), which lowers AFOV vs RFOV or by stretching objects near the field edge (high pincushion), which increases AFOV vs RFOV

All that's required for long eye relief and a wide apparent field is a wide enough eyelens. The afocal light that emerges from the eyepiece forms a cone with the eyelens as the base and the eye relief distance as the height. The angle formed by the curved side of the cone is the apparent field angle. The cone may be large (long eye relief) or small (short eye relief) without having any effect at all on the true apparent field as long as the angle of the curved side is constant.

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Old Wednesday 4th September 2019, 00:18   #6
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We have some issues of theory vs practice here.

Yes, there's potential confusion between distortion and curvature, but in existing bino models they're strongly correlated. (Those I've seen anyway... are there flat field models with classic pincushioning?) I said "flat field" because it's a widely used term; people don't tend to talk about distortion separately and it gets complicated. Is there more convenient terminology than low pincushion distortion, or angle condition vs tangent condition?

Similarly, when I said I couldn't see how ER could be indefinitely increased without affecting field of view, I was envisioning the size of the bino remaining essentially similar, as actual models tend to. I do see how a much larger eyepiece could accomplish more, but in practice there does seem to be an effective trade-off.

I really don't want to try astro binos or even the WX, because just as Peter and others have said, it would only lead to dissatisfaction with anything practical to carry!
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Old Wednesday 4th September 2019, 00:27   #7
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If the tube size and magnification stays the same, you would need a wider ocular with the same focal length (magnification = f_objective / f_ocular), which means its bending the light more, so likely adding curvature, coma, or spherochromatism.

Also, the objective controls the true field of view along with the ocular's field stop radius. A wider field stop radius would likely make it easier to have a larger true FoV because the ocular wouldn't need to bend the light as much.

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Old Wednesday 4th September 2019, 01:44   #8
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Tenex,

It has been explained well already - high ER designs don't necessarily limit the real Fov provided the ocular and glass size is increased to do the job. Also, Henry is spot on - flat fields correct field curvature not necessarily always being low pincushion designs.

As far as what 'we' (given that it is a highly individual thing) 'feel' is an 'immersive' view - well that will entail a couple of factors:-
* the Fov (compared to our subjects behaviour), the Afov (in comparison to our eye's physiology ~60 or more)
* the sharpness throughout and particularly at the edge of the field
* the field curvature in relation to our own eye's accommodation
* the degree of stereopsis (spacing of objectives relative to ocular, and the percentage effect dependent on one's ipd)
* the amount of pincushion distortion and it's profile throughout the field
* the randpupille of the design - ie. how much latitude is there for movement of the eye off-axis before the field is reduced
* the appropriate Eye Relief, and the criticality of that
* the control of stray light, and the level of contrast
* the colour rendition and how closely that matches our unaided eye's perception of the scene
* the transmission levels, or brightness, and again how that matches our unaided eye's perception of the scene
* the 'clarity' of the prisms - 100% internally reflecting prisms (Porro I or II, A-K, or Perger) seem to exhibit a 'clarity' or transparency that Dielectric Mirror coated S-P prisms just don't seem to entirely have despite the losses in the mirror only being circa ~1% 'transmittance'
* the depth of field of the binocular) largely linked to magnification, and the margins of error already listed
* the lack of other distractions drawing the mind's attention - such as poor ergonomics, poor mechanics - focus wheel freeplay, hysteresis etc
* the ipd (Interpupillary distance) being set correctly for one's eye spacing, and facial symmetry
* the collimation of the binoculars and interplay with viewing distances (affects Porros more), and the margins of error within the binocular system that this affects.
* the intrusion of outside light sources (such as eyeglass wearers get with the side light that enters the gap between the eye and glasses

Whether a binocular feels immersive is due to the magical combination of these factors that precisely suit the individual. As we're all pretty much made of the same stuff and put together in a similar way, I think we may be able to agree on a general 'formula' for immersion.

That is a wide enough AFov (~70+) with as much field curvature as we can accommodate without causing blur, and slight pincushion (at a level that is not distracting ~ somewhere around the circle of condition at the extreme edge with a gently increasing distortion profile from the centre). Sufficient ER with a good margin of error (well controlled spherical aberrations). All the other factors I mentioned well taken care of.

Examples of bins I find closest to 'immersive' are the Swarovski 10x50 SV, the Zeiss 8x42 SF, and the Swift Audubon 8.5x44 ED Porro (though that one is without eyeglasses on to get the full Fov). There are druthers with all of them - I don't think my perfect view has been invented yet .....

If I see an immersive view with my eyeglasses on (mandatory ! :) , I find that when I take them off and adjust the binocular accordingly, that the level of immersion seems to increase. I wonder then, if for you it is a matter of setting the ER (and perhaps even the Interpupillary Distance too) more precisely ..... ?




Chosun

Last edited by Chosun Juan : Wednesday 4th September 2019 at 09:48. Reason: clarity
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Old Wednesday 4th September 2019, 03:27   #9
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Originally Posted by Chosun Juan View Post
Tenex,

If I see an immersive view with my eyeglasses on (mandatory ! :) , I find that when I take them off and adjust the binocular accordingly, that the level of immersion seems to increase. I wonder then, if for you it is a matter of setting the ER (and perhaps even the Interpupillary Distance too) more precisely ..... ?

Chosun
Thanks Chosun, For offering up such a comprehensive list of variables. Well done.

I think your last comment that I quoted is what divides much of our perceptions. Binoculars that suit folks that wear eyeglasses, may not deliver an equally immersive result for those that don't, and vice versa. You're lucky if you can get a great view with, and without glasses. I think there's an accommodation window in some binoculars where one is either sitting in the first 10 rows, the middle of the theater, or the back, perceptually speaking with regard to the FOV. In my book, closer is always better, as long as one can see the entire field. Because of that, I do agree with you that the view is generally better without glasses, but that's not going to work for me.

Also, anyone who's used telescopes in the last 20 years or so has seen the evolution of eyepieces into these enormous glass monsters that offer amazing wide apparent fields at a range of focal lengths, allowing low to extremely high magnification, with decent ER, the polar opposite of the older style eyepieces, where one had to practically look through a pinhole to see a high powered view...

There may still be room to adapt and refine the optical envelope of what is practical and desirable in hand held binoculars (without batteries), with reasonable weight. An immersive view is worth a few extra ounces, I'd say.

-Bill
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Old Wednesday 4th September 2019, 05:07   #10
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Perhaps I shouldn't have used the word "immersive" because it can call to mind so many other factors, as Chosun has listed. What I'm interested in here is simply the width of the PFOV ("perceived field of view"), although I can't think of a way to measure it directly. I don't think it's simply a function of magnification and RFOV. (Though perhaps some of the other factors affect it by limiting RFOV.)

Interestingly there is an older thread entitled "PFOV concept", just pointed out by Renze in another thread:
https://www.birdforum.net/showthread.php?t=220415
but the OP is also using words like "walk-in" and "transparency", and FOV is only one of five factors suggested to be involved. (Another is actually "great ER", quite counter to my own impression.) And people are talking about leaving the eyecups down because they cause a tunnel effect, which makes a bino too hard for me to hold in front of my face. And so on.

So I'd like to get back to a simpler concept of PFOV: how wide does the view look (what subjective angle does it subtend), and what factors affect that, in the trade-offs that binos are all about? I think they include angular distortion and ER (directly or indirectly), as previously suggested.

Considering how much binos reduce our FOV to begin with, the remaining question might boil down to this: Could a bino with 20mm ER offer a wider PFOV if it had 15 or 13mm ER, at the same size and cost? (Without eyeglasses of course.)

Last edited by tenex : Wednesday 4th September 2019 at 05:32.
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Old Wednesday 4th September 2019, 10:11   #11
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Bill & Tenex,

I don't think it matters whether a suitably large AFov design is long ER or short ER for getting an immersive view - it only affects the amount of glass required in order not to truncate the field, and hence weight. We don't need to increase the weight of current binoculars to get wide fields and immersive views - what we need is greater materials and mechanical design sophistication to drop the weights from where we find them currently while having sufficient glass to give wide fields.

What matters with regard to ER is that it is able to be adjusted to suit you. If you are unable to get the eye positioned correctly you will either get reduced field, larger expanses of black surrounds, or blackouts /kidney beans etc depending on the design.

When I say that I find the view more immersive without glasses on, this means that I back the eye cups out, and yet if I position my eye at the correct distance I get no increase in the black surround - ie. the Fov takes up the entire view. The extra feeling of immersion (provided it is a good sharp 66+ AFov) comes from not having the side glare from behind my glasses from side light, and a greater degree of alignment margin of error making for an easier view.

Conversely, my eyelashes fluttering up against the eyecups feels a bit weird ......

The NoctiVids went to great design lengths to get that perfect mix of field curvature and pincushion and glare resistance that provides a pleasing view to the eye. I think there is pretty wide consensus in their success of that. The only major factor missing from the NV having an "immersive" view is the lack of more AFov.

PS. I knew I would forget one or more factors in my previous post - so I went back and added 'clarity' too



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Old Wednesday 4th September 2019, 12:02   #12
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You might also consider binobandits or other side shields to remove straylight and other “out the side of your eye” distractions. Can really help connect you to the scene. Can get a bit humid on hot days, but then just fold them down. People are thinking of making wider fields bins, but making ultrawide angle eyepieces in suitably long focal length for low power is a big ask. Usually ends up with huge eye lenses that can impede node access.

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Old Wednesday 4th September 2019, 16:29   #13
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Considering how much binos reduce our FOV to begin with, the remaining question might boil down to this: Could a bino with 20mm ER offer a wider PFOV if it had 15 or 13mm ER, at the same size and cost? (Without eyeglasses of course.)
The answer is "no". Moving the eye closer or farther from the back of the eyepiece does nothing to the true AFOV any more than it changes the size of objects within the AFOV. It seems to me that what you call a higher PFOV means replacing the blackness outside the eyepiece field stop with some sort of of lateral light, however unrelated it may be to the image within the field, something like the way TV news stations add blurry borders outside vertical cell phone videos just to fill the horizontal screen.
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Old Wednesday 4th September 2019, 20:36   #14
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There are a few binoculars, such as very small Minolta flat pack binoculars that have rectangular masks in front of the eyepieces to give a letter box view.

This doesn't influence me one way or the other although it does reduce the actual area shown.

In movies the two circles of a binocular are usually spread across the frame.
There are very few movies where a binocular view is shown as a circle.

B.
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Old Thursday 5th September 2019, 02:33   #15
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I like the OP's distinction between actual, apparent, and perceived FOV. I do think the term immersive opens a broader can of worms as others have pointed out. And, perception is a markedly subjective thing as are all first person experiences.

I had some astronomy experiences this past weekend using eyepieces with apparent field of view (AFV) of 50, 68, 82, and 100 degrees. All were of similar quality (Televue). I would say that in the 50 degree eyepiece the view is slightly cramped or claustrophobic. I feel as if I am looking through a pipe. With 68 or 82 degrees I feel unaware of the field stop of the eyepiece unless I specifically scan to the edge, so the view seems more open. With the 100 degree AFV (17mm Ethos eyepiece), not only was I not aware of the field stop, but it was a bit difficult to find. It felt like I had to stick my head into a hole and look around.

I also viewed through a binoviewer using two eyepieces with 68 degree AFV..The binoviewer just splits the light from a single telescope objective into two eyepieces. The perceived field of view using both eyes is greater, than looking with one eye using the same eyepiece in the same telescope. This, even though the apparent field is the same and the actual field is slightly smaller due to longer focal length of telescope + binoviewer compared with telescope alone.

I've had the same experience with binoculars, that my perceived field of view depends largely on AFV, though as Chosun said, the subject matter and activity matters. In binoculars the break point is around 60 degrees AFV for me. The widest I've used is about 70 and the narrowest just less than 50. I have a 7x50 with 52 degree AFV and a 9x45 with 69 degree AFV. The actual field seen in both is quite similar, but my perception is that the field is wider and more open in the 9x45. I have an 8x32 with a 60 degree AFV again giving me about the same actual field as the other two. My subjective perception is that the 8x32 and 9x45 are similar and more expansive than 7x50. To me, immersive is the extent to which I feel as if I am in the scene, rather than looking at it through an instrument. The 9x45 is most immersive, followed by 7x50, and then 8x32. My sense of immersion seems more to do with over all optical quality rather than any particular optical specification.

Alan
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Old Thursday 5th September 2019, 16:53   #16
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Photographs are oblong.
T.V. and computer screens are oblong.

So why do we accept binocular fields that are round?
Why don't we demand oblong fields with binoculars?

This may be learned behaviour, and the ability to accept different things.
But it seems to be psychological.

I have about one hundred photos on display in frames.
Only one is oval. A portrait just over a hundred years old.
All the rest are rectangular.

The original Kodak box camera images were circular, perhaps 1880s?
There were a few other cameras producing circular images, but the vast majority produce rectangular or square photos.
Our round camera lenses produce oblong images.
Yet our round binocular lenses produce round images.

Why did we say that some people were square, meaning old fashioned?

When I look around, some things in nature are round.
The Moon. Apples etc.
Man made things are often rectangular, but sometimes round depending on usage.
Square whisky bottles break when dropped. Round ones don't.
So round is usually stronger.

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Old Thursday 5th September 2019, 18:20   #17
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The answer is "no". Moving the eye closer or farther from the back of the eyepiece does nothing to the true AFOV any more than it changes the size of objects within the AFOV.
I can't tell whether you're envisioning playing with a single bino here, or imagining different ones with different ER, which is what I'm talking about.
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It seems to me that what you call a higher PFOV means replacing the blackness outside the eyepiece field stop with some sort of of lateral light...
No, I'm surprised how difficult it's turned out to be to clarify what I don't mean. All I care about here ("PFOV") is how much of my visual field the bino view seems to occupy. That actually sounds fairly simple to me. AFOV (as conventionally defined) accounts for most of that, but not all; other factors are involved also, like angular distortion. Is ER another, at some point? Does it tend to reduce PFOV only indirectly, because in practice real FOV is sacrificed to get high ER without making oculars too large and expensive, or is there some direct geometrical effect as well? (Intuitively, it doesn't seem that ER can be increased indefinitely.) Or is my impression that high ER correlates negatively with PFOV somehow mistaken in the first place, based as it is on experience of only a modest number of models?
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Old Thursday 5th September 2019, 18:39   #18
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Originally Posted by Binastro View Post
Photographs are oblong.
T.V. and computer screens are oblong.

So why do we accept binocular fields that are round?
Why don't we demand oblong fields with binoculars?

This may be learned behaviour, and the ability to accept different things.
But it seems to be psychological.

I have about one hundred photos on display in frames.
Only one is oval. A portrait just over a hundred years old.
All the rest are rectangular.

The original Kodak box camera images were circular, perhaps 1880s?
There were a few other cameras producing circular images, but the vast majority produce rectangular or square photos.
Our round camera lenses produce oblong images.
Yet our round binocular lenses produce round images.

Why did we say that some people were square, meaning old fashioned?

When I look around, some things in nature are round.
The Moon. Apples etc.
Man made things are often rectangular, but sometimes round depending on usage.
Square whisky bottles break when dropped. Round ones don't.
So round is usually stronger.

Regards,
B.
Well, a camera throws away a lot of image to make that rectangular view. The lens projects an image circle, not a rectangle, but the film or sensor is what makes it rectangular. You could put some rectangles over your objectives and throw away that part of the view, but I don't think anyone would want that.

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Old Thursday 5th September 2019, 19:08   #19
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Marc,

Having rectangles over the objectives still gives a circular view.

They would need to be put over the eyepieces.

But I think that accepting circular binocular images is learned behaviour.
It doesn't answer the question of why we find the circular view acceptable.

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B.
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Old Friday 6th September 2019, 21:31   #20
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I think we like an elongated or rectangular view for evolutionary reasons: scanning the horizon for prey or predators. But now that we're atop the food chain and the rest is at the grocer's, we can accommodate a circular view. (Do take breaks to watch out for the occasional bear though...)
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Old Wednesday 11th September 2019, 02:28   #21
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Photographs are oblong.
T.V. and computer screens are oblong.

So why do we accept binocular fields that are round?
Why don't we demand oblong fields with binoculars?


B.
Excellent question! Be patient and look up my name in USPTO's published patent application records in 2021..

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Old Wednesday 11th September 2019, 19:28   #22
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Each eyeball has more or less circular entrance pupil aperture, but visual perception is a brain process, constrained by but not overly determined by optics. I think of the retina as an extension of the brain rather than an input device. Past experience and expectation informs what is "seen."

My sense of my visual field without optical aid is that it is wider than it is tall (more azimuth less altitude). Also, my ability to converge an image in binoculars is much more sensitive to altitude miscollimation than Azimuth miscollimation. Though I haven't tested it in detail, I believe it is easier for most people to tolerate Azimuth miscollimation when the tubes are slightly crossed rather than slightly "de-crossed" (no idea of the right term).

My only point with this last bit about collimation is that our perception and perceptual preferences are more a matter of how our brain and various muscles have evolved. Prey animals tend to have different visual and attentional modes than predators
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Old Thursday 19th September 2019, 01:21   #23
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I have a hazy idea of what I think the OP is trying to get at, but struggle to properly articulate it. I noted the other weekend that when comparing my two favourite West German 8x30 porros the Binuxit had a greater "perceived field of view" than the Zeiss West. Stated field of view was identical (150m at 1000m) and this was borne out when checked against landmarks - but I could bring the Leitz a little closer to my eyes without experiencing blackouts, moving the field stop further outward. The Zeiss West had a little (only a little - but noticeable) more tunnel effect. Both of these are, of course, short eye relief designs - I have not yet tested a binocular with really long eye relief (say 20mm) without glasses but the eyecups of such binoculars probably need to be extended a fair bit to avoid blackouts when they are used without glasses, giving somewhat of a tunnel effect (less perceived field of view) even though the user is seeing the full field of view. When using an 8.5x42 SV (stated eye relief 20mm) with glasses I have to extend the eyecups about halfway to the first stop - this does not seem to decrease perceived field of view compared to binoculars with 14 to 15mm eye relief, but if I were using it without glasses I would have to extend the eyecups much further. I'll check this with my brother's unit at the earliest opportunity.

Sharpness all the way to the edge, to me, increases perceived field of view. Binoculars with field flatteners in some cases (eg. Swarovski SV series) have a field of view that is almost all sweet spot and can (at least in my perception) match larger fields of view with poorer edge sharpness.
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Old Thursday 19th September 2019, 02:39   #24
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I have a hazy idea of what I think the OP is trying to get at, but struggle to properly articulate it.

......

Stated field of view was identical (150m at 1000m) and this was borne out when checked against landmarks - but I could bring the Leitz a little closer to my eyes without experiencing blackouts, moving the field stop further outward.

.........


Sharpness all the way to the edge, to me, increases perceived field of view. Binoculars with field flatteners in some cases (eg. Swarovski SV series) have a field of view that is almost all sweet spot and can (at least in my perception) match larger fields of view with poorer edge sharpness.
Patudo,
You've articulated your experience relative to the matter at hand very well.
And your last point has not been made in this discussion (at least recently).

The aesthetic criticisms about flat field that I've read numerous times, do not make much sense to me. In general, I agree with the concept that our eyes tend to focus in the center of the field, so absolute sharpness to the edge isn't a necessity, but I am puzzled by the complaints about it. If it exacerbates rolling ball, that's a separate issue from the actual image itself.

-Bill
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