Binocular Evolution II: Curvature and Distortion (3 Viewers)

[Charlie Yardbird]

So might it be that the wide angle binoculars which are said to have the best "WOW" factor for astronomic views of the night sky, will be found to have the visual distortion parameter "l" close to 1, providing a deep 3D effect?
holgermerlitz.de/globe/distortion.html -- the actual URL is missing the "https://www/" portion, which malformation seems to be making the boards expansion function choke:

Distortion and globe effect in binoculars

Distortion and globe effect in binoculars
by Holger Merlitz

There is another interesting fact to be noticed: Some of the test persons reported on the impression of an apparent image curvature. They observed a convex curvature in case of the tangent condition (k=1) and a concave curvature in case of the angle condition (k=0), while the circle condition (k=0.5) and the k=0.7 checkerboard appeared almost flat. This can be interpreted as an intrinsic curvature of our visual space, that is compensated through the binocular when using a pincushion distortion of about k=0.6.

Such an intrinsic, convex curvature would explain another daily life experience: Why does the curvature of the night sky not resemble that of a half sphere? It seems to have a concave shape, but appears to be somehow flatter than a semi-sphere, rather looking like the inside of a flat bowl, or a wok. In fact, if our visual distortion parameter were equal to unity, l=1, then we would get the impression of a perfect semi-sphere, and in case l=0 the sky might appear flat. A value somewhere in between, e.g. l=0.6, would very well explain our perceived curvature of the night sky.

 

[Charlie Yardbird]

Is there any test equipment that can be hooked up to binoculars and input into video analysis software, which would then calculate k, or the parameters of the moving ball when panning, for that instrument?

 

[kimmik]

Looking at the Noctivid field curvature today I noticed that hidden at 70% ish outer field, the ring nebula of wetzlar? Near the field stop is tack sharp once in focus, just like EL, but the residual curvature hides the ring better.

 

[kimmik]

My drawing comparing 8x telescope with rectilinear magnification vs angular magnification.



The left, rectilinear magnification, combines flat field with angular magnification distortion that maps to a flat image. Ed previously suggested the "flat card" effect.

The right, angular magnification, is usually combined with field curvature (focal plane curvature). This leads to the natural appearance in e.g. leica ultravid - edge is focused closer than centre, and simultaneously appears closer than centre.

 

[Charlie Yardbird]

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.

All four of those diagrams look exactly the same to me.

 

[Binocollector]

Is there any test equipment that can be hooked up to binoculars and input into video analysis software, which would then calculate k, or the parameters of the moving ball when panning, for that instrument?

I'd contact Holger about that as he has some elaborate diagrams on his website and I think most of the analysis was done by the Swaro lab.
He mentions in his article about the distortion profile of the Komz BPO that Swaro did the measurements:

Case study: The distortion of the BPO 7x30

Case study: The distortion of the BPO 7x30

www.holgermerlitz.de

So I'd guess there might be something like the equipment you mentioned.

 

[Holger Merlitz]

This is an interesting thread with lots of technical discussions. Let me add what I believe to know/guess about the recent historical developments in the distortion layout of modern binoculars:

It was actually not Swarovski, but Nikon with its early HG models who started the trend of field-flattening their binoculars. In late 2009, Swaro then followed up with the 8.5x42 EL SV. Shortly thereafter, discussions came up about the unnatural panning behavior of this binocular and I got in contact with them. They were mostly unaware regarding the possible side-effects that the distortion curve could generate during panning. They designed these field-flattening lenses very aggressively to achieve an image that was virtually sharp to the edges, and the resulting distortion curve was just a by-product of these efforts. In computer animations we then compared the panning behavior of several distortion curves, including different degrees of visual distortion, and in this way we worked out which shape of the distortion curve would probably appear least disturbing. The curve should be parabolic, with a gradually increasing pincushion distortion from the center to the edges. It should not turn flat or even bend backwards toward the edge (mustache-distortion), since that would make the image appear uneven during panning, with a wave-like bump in its outer areas. Unfortunately, it is not easy to achieve both, a very flat field to the edge and a smooth distortion curve. Zeiss seems to have chosen a less aggressive approach to field flattening and thus mostly avoided these side-effects (although with the 8x42 SF there seems to be a little bit of a mustache-effect, too). Leica has been even more conservative: They implement a moderate field flattening, leave the edges blurred and keep a nice parabolic distortion curve.

Nikon with the 10x50 WX has shown that it is possible to flatten the field to the edge and at the same time to keep the distortion curve parabolic. Their patent documents contain on pages 41 and 42 the eyepiece design which is probably very close to the one used in the commercial 10x50 WX. It contains a very complex field-flattening unit made of 4 lens elements, and this is probably the way to go. Since it is the field flattener which bends the distortion curves, a very sophisticated design must be chosen to avoid these negative effects. The 10x50 Nikon WX has a very wide subjective angle of field, about 84°-85°, yet sharp edges, and a fine and smooth panning behavior without any globe effect. The 7x50 WX probably uses the eyepiece design of page 37 and 38, with less pincushion distortion. It shows a very slight globe effect, but since its subjective angle is just about 70°, it is not that disturbing. Nikon 10x50 WX designer Katsunori Tomita describes in his interview that "Due to the binoculars' wider apparent field of view, issues arose that the design software seemed unable to anticipate and that could only be seen for the first time during actual viewing." I am quite sure that these 'issues that the design software seemed unable to anticipate' included the panning behavior, which is of dramatic importance with such wide angle instruments. The 10x50 had to be re-designed before it could finally go into production, but now it is really perfect.

Cheers,
Holger

 

[kimmik]

Dear Holger, looking forward to your new book.

Any thoughts on how many/types of aspheric surfaces are present in e.g. the WX, SF etc?

 

[tenex]

Let me add what I believe to know/guess about the recent historical developments in the distortion layout of modern binoculars...

Thank you. So complex distortion profiles are indeed a byproduct of aggressive field-flattening, rather than a desirable new approach to distortion effects for optimal viewing. That's what I suspected and wanted to confirm when I began this thread.

 

[henry link]

Thank you. So complex distortion profiles are indeed a byproduct of aggressive field-flattening, rather than a desirable new approach to distortion effects for optimal viewing. That's what I suspected and wanted to confirm when I began this thread.

No, that is not true. Compound distortion may accompany field flattening, as in the case of Swarovski's designs, but it is not an inevitable by-product, nor is it necsessary. I have about ten optical devices with field flatteners. Their distortion profiles run the gamut from extremely high pincushion (with reversed AMD) to essentially zero pincushion (with very high AMD.) Only one uses compound mustache distortion.

 

[Binastro]

Did Canon use binocular field flatteners before Nikon?

Regards,
B.

 

[henry link]

The first field flattener I encountered was in the Nikon 7x50 SP about 1983. The Nikon 8x40 Classic Eagle came a few years later. Fujinon FMT-SXs were introduced around 1985, the Pentax PIFs in 1993. The Nikon HGs came out around the mid 1990s. I think the first IS Canons with field flatteners came out in the late 1990s or around 2000.

Swarovski was a "pioneer" in field flatteners only among the European brands, which were 25-35 years behind the Japanese.

 

[kimmik]

I have about ten optical devices with field flatteners. Their distortion profiles run the gamut

Is it possible to narrow down the root cause of compound distortion?

I imagine external focus (moving eyepiece or objective), eg the WX and some others, makes correction of all aberrations easier.

Is there a weight difference? FOV difference?

Any compound distortion bino, if you crop out the outer bit, will no longer be compound distortion. So thats one reason the compromise exists - wider fov.

 

[Binastro]

It seems that a Smyth field flattener lens was used for photography about 1874.

Regards,
B.

 

[tenex]

No, that is not true. Compound distortion may accompany field flattening, as in the case of Swarovski's designs, but it is not an inevitable by-product, nor is it necsessary. I have about ten optical devices with field flatteners. Their distortion profiles run the gamut from extremely high pincushion (with reversed AMD) to essentially zero pincushion (with very high AMD.) Only one uses compound mustache distortion.

If you read my post again you will see that nowhere did I say compound distortion was an inevitable or necessary byproduct, only that it is a byproduct rather than a deliberate result. It is possible (but challenging as Holger points out) to avoid; it has seldom been avoided, at least in binoculars under $6k. If other "optical devices" do better one could wonder why.

 

[henry link]

...it (compound distortion) has seldom been avoided, at least in binoculars under $6k. If other "optical devices" do better one could wonder why.

No, overall its compound distortion that is rather seldom used rather than seldom avoided and cost has nothing to do with it. The designers of other flat field "optical devices", meaning other flat field eyepieces, have simply chosen other distortion profiles. That's not necessarily doing better, it's just making a different choice. My guess is that compound mustache distortion was chosen by Swarovski and then Zeiss simply to avoid the appearance of too much old fashioned pincushion distortion in the image by taking most of it away near the edge of the field. As Holger mentioned, the guys at Swarovski were apparently unaware that low pincushion at the field edge would distort shapes there in a way that leads to the globe effect when panning.

 

[Maljunulo]

I bought a Fujinon FMT-SX (yellow ring) 10X70 when they first came out, and gave my old ones to my then Son-in-Law.

 

[Holger Merlitz]

Dear Holger, looking forward to your new book.

Any thoughts on how many/types of aspheric surfaces are present in e.g. the WX, SF etc?

Thank you, Kimmik! As far as I know, there are no aspheric surfaces employed in the WX or SF. They would be costly to make and offer little advantage over modern designs with lots of lens elements.

Cheers,
Holger

 

[Holger Merlitz]

No, overall its compound distortion that is rather seldom used rather than seldom avoided and cost has nothing to do with it. The designers of other flat field "optical devices", meaning other flat field eyepieces, have simply chosen other distortion profiles. That's not necessarily doing better, it's just making a different choice. My guess is that compound mustache distortion was chosen by Swarovski and then Zeiss simply to avoid the appearance of too much old fashioned pincushion distortion in the image by taking most of it away near the edge of the field. As Holger mentioned, the guys at Swarovski were apparently unaware that low pincushion at the field edge would distort shapes there in a way that leads to the globe effect when panning.

Yes, Swaro's highest priority at that time was a perfect edge-sharpness and apparently they didn't pay enough attention to the side-effects of the resulting distortion curve. I wonder whether Zeiss has run into the same problem and for that reason avoided the implementation of such a perfect edge-sharpness. It is definitely possible to design eyepieces with high edge-sharpness and any thinkable distortion curve - theoretically, these are independent parameters, once there are sufficient degrees of freedom available to the designer. But there are restrictions that come with a common birder's binocular (compact and light eyepieces) which could lead to these undesired correlations between field flattening and weird distortion curves, at least with wide-angle eyepieces. A Nikon WX or Nagler eyepiece may solve these problems easily, but for that they need lots of glass, which is no option for a compact binocular.

Cheers,
Holger

 

[Binocollector]

I wonder when the Komz 7x30 with its complex eye-piece was first introduced. My "Sekonda" is from '91. I also have the newer version that lacks the internal focusing. But it's just as good as the old one.
I also tried to find out when the (flat field) Canon 8x32WP was made. I think around 20 years ago. It has a very similar distortion to the Komz with a slight mustache distortion.
I think the Canon is still excellent if only it had p-coatings it would be even better. It is a bit on the dim side but still razor sharp.