Binocular Evolution II: Curvature and Distortion (1 Viewer)

[William Lewis]

What I found interesting in holger's article (amongst many) was the way the SLC 56's were spread so widely accross their tendencies for different distortions. I assume that's because the eye lenses are the same most of the way through with the very last lens being changed to cater for the different magnifications, hence the 10x being in the sweet spot, the 15 having more rolling ball and the 8 having more pincushion etc. As I personally dislike rolling ball it works for me, I'll have to take some snaps of straight lines or circles to see how bad they are- as mentioned I tend to look at birds more often than not!

 

[tenex]

What I found interesting in holger's article (amongst many) was the way the SLC 56's were spread so widely accross their tendencies for different distortions.

Nice observation. I'm not sure whether the small real field somehow mitigates the RB propensity of the 15x56, or I'm just not very sensitive to it.
By the way, a later version of Holger's graph also includes SF32 and NL42:
(NL8x42 is an interesting outlier, and the model where odd edge effects have been most noted, perhaps struggling a bit)

Complex/second order distortion is probably not intentional

the field flattener is the most likely culprit for the complex distortion in EL series.

This is really what I've been suspecting, and getting at in too roundabout a way. Those who speak carelessly about "flat field" here are regularly admonished to distinguish between curvature and distortion effects as if the term referred to one and not the other, but they don't occur independently. And we should clarify the full consequences of a field flat to the very edge, so people don't just say "Well, why not have it?"

A question remains about the first-order effect. In Swarovision, is the overall low pincushioning itself a deliberate choice to reverse a historical trend, or does field flattening just require it?

 

[Binocollector]

It was an independent design choice. It's all in the article from Holger that I linked. Pincushion distortion was introduced to reduce the rolling ball effect and has nothing to do with field flattening.
Curvature and pincushion distortion are two different things -- each of those terms describes a different phenomenon. That doesn't mean though that an extra lens element to reduce the curvature can not also influence the distortion I guess. And with any extra element there are probably new problems and distortions.

 

[William Lewis]

So given the SLC 8x56 is the worse on the graph, here's how it looks in practice. And you get the joy of admiring my wife's lovely oven gloves. Looks terrible, never noticed it in use!

 

[William Lewis]

And just to dive a little deeper into the rabbit hole here's an image through the srga 8x32. Note no field flattener, similar distortion profile...

 

[kimmik]

Holger's graph is a useful overview but the 8x42 NL is almost certainly calculated assuming 8x as the true magnification, when it is actually 8.2x, thus erroneously placing the distortion profile closer to angle condition. Visually it has completely uniform mild pincushion distortion, while the EL 8x32 has the moustache at the periphery.

I tried to photograph my findings but it turns out the iphone camera has mild barrel distortion when photographing grid pattern 😂 so it would just confuse things seeing how we're delving so deep into this.

William good pictures for putting the graph findings into perspective.

 

[Binocollector]

I tried to photograph my findings but it turns out the iphone camera has mild barrel distortion when photographing grid pattern 😂 so it would just confuse things seeing how we're delving so deep into this.

Good point. I think I need to take a pic with my phone camera of a grid pattern as well. I often think mine adds pincushion distortion. The whole pincushion distortion is only neccessary anyways to counter the barrel distortion of our eyes which is what causes the rolling ball effect in a bino with perfectly straight lines. So a picture taken through the binos will probably show a worse pincushion distortion than when actually looking through it.

 

[kimmik]

A question remains about the first-order effect. In Swarovision, is the overall low pincushioning itself a deliberate choice to reverse a historical trend, or does field flattening just require it?

My own reasoning is that angular condition encourages binocular panning and less eye panning, while rectilinear condition encourages eye panning with less binocular panning.

	Curved field + edge falloff	Flat field + sharp nearly to edge
High pincushion	e.g. ultravid, noctivid (less so)	doesn't exist?
Low pincushion	doesn't exist?	e.g. NL, SF (based on what i read)

If a bino view is fully corrected to the edge, a person will naturally pan around with their eyes, thus it would make sense to have low pincushion.

If a bino view has a small central sweetspot, then there is no incentive to pan with your eyes, which means more panning with the bino, thus you need angular condition to prevent rolling ball effect.

From a psychology physiology point of view, both leica and swaro/zeiss approach makes sense to me. On the other hand it would confuse me, to have flat field with high pincushion.

So from this angle of analysis, the bundling of low pincushion with flat field is deliberate.

 

[kimmik]

A question remains about the first-order effect. In Swarovision, is the overall low pincushioning itself a deliberate choice to reverse a historical trend, or does field flattening just require it?

My second angle is to look at how flat field is being implemented in the three premium binoculars.

NV - weak negative element without subsequent positive element. this would reduce the field of view/increase magnification by also acting like a barlow lens, and increases eye relief since the effective focal length has increased.

NL - a strongly negative first surface which: flattens the field fully, magnifies the image circle (by lengthening the effective objective focal length), introduces pincushion distortion and other aberrations
- this is followed by a positive meniscus, which corrects the new aberrations. Here is possibly the source of EL's moustache distortion.

SF - strongly negative but biconcave element, followed by positive doublet.

So it seems a weak single element flattener, allows you to keep FOV and pincushion distortion in the SLC range, but necessitate a larger than SLC eyepiece as the effective objective focal length has been increased. Good for eye relief. Also explains why NV is 850g while SLC is 800g.

If you want to flatten further, you will start to lose FOV since we are at max size eyepiece, so you add a positive element after the flattener to bring the image circle back down. This would reduce eye relief, but increase field of view.

Now the crux of Tenex's question is, does this additional positive element, have to remove so much pincushion, from the technical point of view?

I've no idea

 

[tenex]

Visually it has completely uniform mild pincushion distortion

The oddity I recall in NL8x42 is a ring of softness in the outer field. Roger Vine has a photo in his review. Something is being pushed a bit far here.

doesn't exist?

I'm uncomfortable with this chart because it lumps bins that aren't truly "flat field" and show none of the distortion oddities we've discussed, like SLC (though not listed), in with others that are and do. Not sure whether the solution is a new middle column, or putting SLC etc in the left one.

If a bino view is fully corrected to the edge, a person will naturally pan around with their eyes, thus it would make sense to have low pincushion.

But everyone ultimately pans bins at some point, so will be bothered by RB (perhaps less often) and why not avoid it. I don't see a good argument for low pincushioning, but it certainly is curious to have no examples of conventional pincushioning + flat field.

 

[kimmik]

Ah yes the absam ring nebula, a quirk reminding us how hard things are working under the hood.

My table was to illustrate the two axes of optimisation, not actually intended to categorise into four strict quadrants. most binos seem to sit on a sloped line like main sequence stars.

 

[kimmik]

These four ray trace diagrams I've recreated the topology of NL/EL/Noctivid field flattener:

1 - with only the negative element, leading to high pincushion (angle condition)

This is the Noctivid flattener. The eyepiece then needs barrel distortion to reduce the pincushion.

2 - with flattener followed by positive meniscus, leading to perfectly parallel rectilinear image (severe rolling ball).


3 - meniscus too small, leading to rolled edge (this would seem to be the case for the EL 8x32)


4 - larger but less strong meniscus, leading to in-between distortion and no rolled edge (NL)

Simulator - Ray Optics Simulation

A free, open-source web app for creating and simulating 2D geometric optical scenes.

ricktu288.github.io

 

[kimmik]

Separate thread for further depth.

Ray Optics Simulation of Swarovski NL, SLW

To help me understand the optical design of the NL series, I have approximated the design using the online simulator linked below. The design file is attached so you may do your own modifications, including changes to the field flattener lenses, to see its effect on field curvature, field of...

www.birdforum.net

 

[tenex]

Curvature and pincushion distortion are two different things -- each of those terms describes a different phenomenon.

This is the standard pedantic (no offense, it's just the right word) argument that I outlined in the OP and wanted to get beyond here, because in practice these phenomena do not vary independently; field flattening affects and has to manage both together.

It was an independent design choice. It's all in the article from Holger that I linked.

Well, he does note a few "experiments" with low pincushioning that appeared on the market in the period 2005-9 (which of course were just such a choice) before the introduction of Swarovision. But it's hard to see EL SV as a mere "experiment" in this vein, as it would have been an incredibly costly and risky one. SV seems really to have been about the totally flat field, and its distortion profile more of a (somewhat problematic) side effect. But I suppose there's always an element of guesswork, for those outside the process.

 

[elkcub]

If a bino view is fully corrected to the edge, a person will naturally pan around with their eyes, thus it would make sense to have low pincushion.

If a bino view has a small central sweetspot, then there is no incentive to pan with your eyes, which means more panning with the bino, thus you need angular condition to prevent rolling ball effect.

From a psychology physiology point of view, both leica and swaro/zeiss approach makes sense to me. On the other hand it would confuse me, to have flat field with high pincushion.

What's been left out of the discussion to this point is that there is also a negative perceptual aspect to flat-field images. It's not as easy to describe in words as the rolling ball (globe) effect, which is a motion illusion, but many people like myself perceive an unnaturally "flat card" type visual field that distorts spatial depth relationships. I am very sensitive to it and liken it to the old-fashioned 3D card images one sees through a stereoscope (shown below).



It is often said, without experimental proof, that panning with the eyes has some benefit over panning with the head. The argument seems to completely ignore that while using binoculars small head motions are magnified on the retina. So, although the head normally moves slower that the eye, the advantage of eye speed is largely lost using binoculars. Second, eye-panning necessarily introduces the need to recenter the exit pupil to minimize vignetting, and third, the head will follow along anyway, as nature intended.

Ed

 

[William Lewis]

What's been left out of the discussion to this point is that there is also a negative perceptual aspect to flat-field images. It's not as easy to describe in words as the rolling ball (globe) effect, which is a motion illusion, but many people like myself perceive an unnaturally "flat card" type visual field that distorts spatial depth relationships. I am very sensitive to it and liken it to the old-fashioned 3D card images one sees through a stereoscope (shown below).

View attachment 1475538

It is often said, without experimental proof, that panning with the eyes has some benefit over panning with the head. The argument seems to completely ignore that while using binoculars small head motions are magnified on the retina. So, although the head normally moves slower that the eye, the advantage of eye speed is largely lost using binoculars. Second, eye-panning necessarily introduces the need to recenter the exit pupil to minimize vignetting, and third, the head will follow along anyway, as nature intended.

Ed

View attachment 1475542

Hi Ed.

Which binoculars do you particularly notice it with? Are there any included in the chart from Holger above?


Will

 

[kimmik]

"flat card" type visual field that distorts spatial depth relationships

I'm becoming more convinced that field flattening/lower pincushion is the most compact way to achieve wide true FOV. The flat card effect is a downside along the way. If we are happy to stay at 7-8degree max FOV, then the roof options are plenty (enabled by ED glass fast objective).

Even though pincushion and field curvature are independently adjustable, it seems on my simulations that they are indirectly bundled together due to the fixed size of eyepieces. So either one accepts strong field curvature, or accept lower pincushion, or lose the 8x option.

Maybe SLW will come in 10x and 12x only (in essence, noctivid with shorter eye relief, wider AFOV)

 

[tenex]

What's been left out of the discussion to this point is that there is also a negative perceptual aspect to flat-field images. It's not as easy to describe in words as the rolling ball (globe) effect, which is a motion illusion, but many people like myself perceive an unnaturally "flat card" type visual field that distorts spatial depth relationships.

I alluded to this toward the end of post #1, but don't seem to be much bothered by it myself, just as I often don't get the "3D" effusions for other models. Do you have a link to past discussion(s), or a theory of what's responsible for it? Perhaps between the magnification and restriction of FOV and DOF, my brain doesn't know what to expect the view to look like and takes it for what it is.

 

[elkcub]

Which binoculars do you particularly notice it with? Are there any included in the chart from Holger above?

Hi Will,

The only one included in Holger's chart is the Swaro 10x42 SV, which I had a chance to compare at length with the (original) 10x42 SLC-HD about 10 yrs. ago. (tempus fugit!) After that I never considered buying another field flattened instrument. Previously, I had owned Nikon 8x32 SE and 8x32 LX L binoculars, but sold them with prejudice in spite of their fine build quality and positive hype at the time. My impression is that once one perceives the flat card effect it simply can't be un-perceived — at least not by me.

As for explaining WHY this perception occurs, or why many people don't perceive it, 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). Beyond that, I'm at a loss for insight as to how "flattened" vs unflattened images modify this relationship, which also varies between instruments as your simulations confirm. Who knows what the optimization criteria actually are?

Tobias Mennle seems to have an extensive understanding of the perception I'm talking about, as discussed in the article below and several others.

Ed

 

[GrampaTom]

With all this understood, further questions arise:

1. What is such complex distortion trying to accomplish? Is it an attempt to offer the best of both worlds, reducing the bending of straight lines near the edge while also minimizing the compression effect to keep panning comfortable? Why then does its profile vary among models, even of the same line? (Not to mention, how is any of this even possible? Surely some very complex aspheric surfaces are needed in the eyepiece.)

2. How did these two things come to be done together in Swarovision? Are they really separate innovations, or is the purpose of the distortion profile somehow related to having sharper field edges? (Does that make distortion effects worse? Did the calculations required for one make the other necessary as well?)

3. Why is this approach taking over the alpha binocular market when many don't need it, and some even dislike it? The RB effect has been reduced in more recent designs, but some are still sensitive to it. Some feel that sharpness throughout gives an unnaturally "flat" view, though it's not obvious why since any binocular offers a much narrower field than normal vision to begin with, and the eye itself has only a limited central area of sharpness that doesn't even cover that. Are they perceiving something subconsciously through peripheral vision? (I'm not drawn to such designs myself and would like to know why, since I don't share these objections and can't point to anything else specific that bothers me.)

I could say more, but possibly that's enough to get a discussion started...

First Tenex thanks for taking the time to write this comprehensive set of history, observations, technical explanations and finally these questions. It is refreshing to get beyond todays fashionable email, text, short one line conversations about things. Some stuff deserves this sort of discussion.

The back and forth between you and kimmik seems serendipitous. That combined with my set of Holger bookmarked articles on Rolling Balls, AMD, etc to include the one posted above in #9, that seemed most relevant to the technical parts of this discussion, has helped me at least to follow along.

Regards the questions posted at the end of the two pieces, I come to a different place answer-wise. I feel like there's two areas of consideration that are not so much addressed by the optical engineering details, that may provide part of the answers you seek.

First, my sense is the direct and explicit answers to these questions belong with the makers. And they're not talking. Looking back at some of the history and thinking about the world that existed when those milestone binoculars were developed, one can only wonder on what was the process employed within the companies that decided to make binoculars with those design choices. Did they hire a "renowned" optical expert of the day, someone who'd done research, maybe taught, published and was known in the optics community? Did a company hire that person and put their ideas of what a binocular should be, into the binocular they brought to market? And that became their signature?

What about today's probably more complex market and evolved businesses? Would there be a design team comprised of a more modern evolved optics engineering group riding on the shoulders of those past greats. Might (probably) that team include representatives from Process Engineering and manufacturing as sometimes a design has to be modified to fit into the available manufacturing processes. Think of the NL, for example. Last, definitely not least, can we suspect the modern marketing/product planning group are not a bunch of innocent bystanders waiting to drop advertising and deploy salesman, but rather have performed sophisticated market research to drive the design choices the companies then believe will reach the most of the market it can. If I have this right then many of the answers to your questions lie with these folks.

Based on the inputs here, we should now understand bino makers were making a choice. The things some folks do not like were not inadvertent mistakes, but the necessary product of the tradeoffs known and made. The fair question left to Swaro, Zeiss. Leica, etc is "What were they thinking!?" Wouldn't we love to know?

Second, Birdforum as we have discussed is a rather exclusive place. It is a quite small group of active participants with an unusual interest in binocular performance. For many here, looking at and studying the binocular is at least as important as looking through them and seeing the the world beyond the glass, magnesium and rubber. Binocular companies need markets bigger than us, need the much broader audience of people throughout the world that just want to see things closer and/or bigger. Do we represent what the larger market sees but does not recognize/understand? Possibly. As an example some here don't get flat field. Some do. As I read your preferences I wonder, are they the product of that 10 years of using those UV1032s with what you describe as conventional design and so Els and NLs represent a difference that you've not yet used long enough to see why? When I look out at the world through the eyes and brain I possess, sans binoculars, I dont see the view described by folks who like that convention. The view I experience, especially in the central better focused area is essentially flat. It does not curve off at the edge. It does get blurry, waiting for the movement of the approaching saber tooth tiger to trigger head and leg motion. Wouldn't I want ny binocular to represent what my eyes see? Whether I'm and eye panner or binocular panner - thank you Kimmik for those phrases - wouldn't a widish field that has a quality view all the way to the edge be useful? If we can get that with all the optical engineering work you and kimmik have described, why not? I dont mean to infer folks who dont like flat field haven't tried it and/or are set in their ways, apologies if it came off that way. I totally respect we each see things the way we do and have our innate differences. It is this complexity of the human component that Im thinking about, regarding your questions.

In these first and second reactions to the questions you ask, Ive posed more, not answered any. It seems the human element and the manufacturing piece are at least as important to providing those answers. The human piece is likely way more complex. The manufacturing choices chase those and yield the answers to why those optical choices, I suspect.